Revert of Remove the rtc_relative_path GYP variable and similar defines

Revert of Remove the rtc_relative_path GYP variable and similar defines (patchset #1 id:1 of https://codereview.webrtc.org/1925733002/ )

Reason for revert:
Breaks downstream gtest usage.

Original issue's description:
> Remove the rtc_relative_path GYP variable and similar defines
>
> This is a reland of https://codereview.webrtc.org/1903553003/ but with
> the SRTP changes removed, since they're needed downstream.
>
> The defines that can be used to alter the include paths for Expat and gtest
> are no longer needed in WebRTC or Chromium. Remove them to simplify GYP.
>
> Removed defines:
> EXPAT_RELATIVE_PATH
> GTEST_RELATIVE_PATH
>
> They're all set in the Chromium build so this shouldn't affect Chromium:
> https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjingle/libjingle.gyp
>
> BUG=webrtc:4256
> NOTRY=True
> NOPRESUBMIT=True
> TBR=perkj@webrtc.org
>
> Committed: https://crrev.com/081254f2c62037d016f9fc961764c6f01cb095da
> Cr-Commit-Position: refs/heads/master@{#12536}

TBR=perkj@webrtc.org,kjellander@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:4256
Committed: https://crrev.com/c6c00b32dabd98962e26efbf6dce9f931076f31e
Cr-Commit-Position: refs/heads/master@{#12622}

[phoglund, 2016-05-04 07:51:29]

Created Revert of Remove the rtc_relative_path GYP variable and similar defines

[commit-bot: I haz the power, 2016-05-04 07:51:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1945803003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1945803003/1

[commit-bot: I haz the power, 2016-05-04 08:02:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1945803003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1945803003/1

[commit-bot: I haz the power, 2016-05-04 08:31:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1945803003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1945803003/1

[commit-bot: I haz the power, 2016-05-04 08:54:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-05-04 08:54:42]

Description was changed from

==========
Revert of Remove the rtc_relative_path GYP variable and similar defines
(patchset #1 id:1 of https://codereview.webrtc.org/1925733002/ )

Reason for revert:
Breaks downstream gtest usage.

Original issue's description:
> Remove the rtc_relative_path GYP variable and similar defines
> 
> This is a reland of https://codereview.webrtc.org/1903553003/ but with
> the SRTP changes removed, since they're needed downstream.
> 
> The defines that can be used to alter the include paths for Expat and gtest
> are no longer needed in WebRTC or Chromium. Remove them to simplify GYP.
> 
> Removed defines:
> EXPAT_RELATIVE_PATH
> GTEST_RELATIVE_PATH
> 
> They're all set in the Chromium build so this shouldn't affect Chromium:
>
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjin...
> 
> BUG=webrtc:4256
> NOTRY=True
> NOPRESUBMIT=True
> TBR=perkj@webrtc.org
> 
> Committed: https://crrev.com/081254f2c62037d016f9fc961764c6f01cb095da
> Cr-Commit-Position: refs/heads/master@{#12536}

TBR=perkj@webrtc.org,kjellander@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:4256
==========

to

==========
Revert of Remove the rtc_relative_path GYP variable and similar defines
(patchset #1 id:1 of https://codereview.webrtc.org/1925733002/ )

Reason for revert:
Breaks downstream gtest usage.

Original issue's description:
> Remove the rtc_relative_path GYP variable and similar defines
>
> This is a reland of https://codereview.webrtc.org/1903553003/ but with
> the SRTP changes removed, since they're needed downstream.
>
> The defines that can be used to alter the include paths for Expat and gtest
> are no longer needed in WebRTC or Chromium. Remove them to simplify GYP.
>
> Removed defines:
> EXPAT_RELATIVE_PATH
> GTEST_RELATIVE_PATH
>
> They're all set in the Chromium build so this shouldn't affect Chromium:
>
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjin...
>
> BUG=webrtc:4256
> NOTRY=True
> NOPRESUBMIT=True
> TBR=perkj@webrtc.org
>
> Committed: https://crrev.com/081254f2c62037d016f9fc961764c6f01cb095da
> Cr-Commit-Position: refs/heads/master@{#12536}

TBR=perkj@webrtc.org,kjellander@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:4256

Committed: https://crrev.com/c6c00b32dabd98962e26efbf6dce9f931076f31e
Cr-Commit-Position: refs/heads/master@{#12622}
==========

[commit-bot: I haz the power, 2016-05-04 08:54:43]

Patchset 1 (id:??) landed as
https://crrev.com/c6c00b32dabd98962e26efbf6dce9f931076f31e
Cr-Commit-Position: refs/heads/master@{#12622}