Relanding Generate localhost candidate when no STUN/TURN and portallocator has the right flag spefi…

Relanding "Generate localhost candidate when no STUN/TURN and portallocator has the right flag spefied."

Migrated from https://codereview.webrtc.org/1275703006/ which causes test failures for android. On android, loopback interface was used as local interface to generate candidates. Add a test case to make sure this won't be broken in the future.

Also observed some failures under content_browsertests in chromium.fyi bot but can't repro locally. Might just be temporary test issue.

BUG=webrtc:4517
TBR=pthatcher@webrtc.org

Committed: https://chromium.googlesource.com/external/webrtc/+/fe3bc9d5aeffed8bbfb34c330d8b991abd1a1aba

[guoweis_webrtc, 2015-08-19 21:14:07]

The CQ bit was checked by guoweis@webrtc.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-19 21:14:20]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1299333003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1299333003/1

[guoweis_webrtc, 2015-08-19 23:50:37]

The CQ bit was unchecked by guoweis@webrtc.org

[guoweis_webrtc, 2015-08-20 04:55:25]

The CQ bit was checked by guoweis@webrtc.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-20 04:55:34]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1299333003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1299333003/40001

[commit-bot: I haz the power, 2015-08-20 05:05:39]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-20 05:05:40]

Dry run: Try jobs failed on following builders:
  linux on tryserver.webrtc (JOB_FAILED,
http://build.chromium.org/p/tryserver.webrtc/builders/linux/builds/9112)

[guoweis_webrtc, 2015-08-20 05:33:38]

The CQ bit was checked by guoweis@webrtc.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-20 05:33:42]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1299333003/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1299333003/60001

[commit-bot: I haz the power, 2015-08-20 07:33:49]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-20 07:33:50]

Dry run: Try jobs failed on following builders:
  android on tryserver.webrtc (JOB_TIMED_OUT, no build URL)

[guoweis_webrtc, 2015-08-20 15:48:33]

Committed patchset #4 (id:60001) manually as
fe3bc9d5aeffed8bbfb34c330d8b991abd1a1aba (presubmit successful).