Simplify BitrateAllocator::AddBitrateObserver.

Simplify BitrateAllocator::AddBitrateObserver.

Remove start_bitrate_bps which is no longer used and return the current
allocated bitrate instead of having it as an out parameter, removing the
previous return value which is no longer used.

Permits removing bitrate controller usage from ViEEncoder.

BUG=webrtc:1695
R=stefan@webrtc.org

Committed: https://crrev.com/8e4e8b045523efb4d4d3416cc17ddb3d1705e41d
Cr-Commit-Position: refs/heads/master@{#9942}

[pbos-webrtc, 2015-09-15 11:14:28]

PTAL

[stefan-webrtc, 2015-09-15 11:20:13]

lgtm

[stefan-webrtc, 2015-09-15 11:20:29]

The CQ bit was checked by stefan@webrtc.org

[commit-bot: I haz the power, 2015-09-15 11:20:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1343783006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1343783006/1

[commit-bot: I haz the power, 2015-09-15 11:29:23]

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

[commit-bot: I haz the power, 2015-09-15 11:29:23]

Try jobs failed on following builders:
  win on tryserver.webrtc (JOB_FAILED,
http://build.chromium.org/p/tryserver.webrtc/builders/win/builds/9661)
  win_x64_rel on tryserver.webrtc (JOB_FAILED,
http://build.chromium.org/p/tryserver.webrtc/builders/win_x64_rel/builds/9344)

[pbos-webrtc, 2015-09-15 12:35:13]

fix test

[pbos-webrtc, 2015-09-15 13:08:18]

Committed patchset #2 (id:20001) manually as
8e4e8b045523efb4d4d3416cc17ddb3d1705e41d (presubmit successful).

[commit-bot: I haz the power, 2015-09-15 13:08:23]

Patchset 2 (id:??) landed as
https://crrev.com/8e4e8b045523efb4d4d3416cc17ddb3d1705e41d
Cr-Commit-Position: refs/heads/master@{#9942}