Add support for config changes during playback to FFmpegVideoDecoder.

Add support for config changes during playback to FFmpegVideoDecoder.
Also fixed a minor config change related bug in SourceBufferStream.


BUG=122913
TEST=PipelineIntegrationTest.MediaSource_MP4ConfigChange, SourceBufferStreamTest.ConfigChange_Basic, PipelineIntegrationTest.MediaSource_Seek

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=152666

[acolwell GONE FROM CHROMIUM, 2012-08-21 21:19:08]

Attempt two.. Forgot to add config_change_pending_ member initializers. Inital
patch set is what was LGTMed in http://codereview.chromium.org/10836304/ . Patch
Set 2 contains the changes.

[Ami GONE FROM CHROMIUM, 2012-08-21 21:20:30]

lgtm

[commit-bot: I haz the power, 2012-08-21 21:20:59]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/acolwell@chromium.org/10828425/2001

[commit-bot: I haz the power, 2012-08-21 23:10:19]

Try job failure for 10828425-2001 (retry) on mac_rel for step "browser_tests".
It's a second try, previously, step "browser_tests" failed.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=mac_rel&nu...