Reland of Use RtcpPacket to send REMB in RtcpSender

Reland of Use RtcpPacket to send REMB in RtcpSender (patchset #1 id:1 of https://codereview.webrtc.org/1300863002/ )

Reason for revert:
This wasn't the cause of the breakage. Re-reverting.
https://code.google.com/p/webrtc/issues/detail?id=4923

Original issue's description:
> Revert of Use RtcpPacket to send REMB in RtcpSender (patchset #1 id:1 of https://codereview.webrtc.org/1290573004/ )
>
> Reason for revert:
> A few bots started failing rtc_unittests after this was commited. Ex https://build.chromium.org/p/client.webrtc/builders/Linux64%20Debug/builds/5048
>
> Original issue's description:
> > Use RtcpPacket to send REMB in RtcpSender
> >
> > BUG=webrtc:2450
> > R=asapersson@webrtc.org
> >
> > Committed: https://chromium.googlesource.com/external/webrtc/+/35ab4baa20a730de71b390008900a16563cbbe8e
>
> TBR=asapersson@webrtc.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:2450
>
> Committed: https://crrev.com/141c5951f4beda868797c2746002a4b1b267ab2a
> Cr-Commit-Position: refs/heads/master@{#9723}

TBR=asapersson@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:2450
Committed: https://crrev.com/dd4edc5813a0331049f53a93ac2404a8899e6ae8
Cr-Commit-Position: refs/heads/master@{#9754}

[sprang_webrtc, 2015-08-21 11:21:42]

Created Reland of Use RtcpPacket to send REMB in RtcpSender

[commit-bot: I haz the power, 2015-08-21 11:21:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1309723002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1309723002/1

[commit-bot: I haz the power, 2015-08-21 11:21:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-21 11:22:11]

Patchset 1 (id:??) landed as
https://crrev.com/dd4edc5813a0331049f53a93ac2404a8899e6ae8
Cr-Commit-Position: refs/heads/master@{#9754}