Revert of Fix seeking when the start time is non-zero.

Revert of Fix seeking when the start time is non-zero. (https://codereview.chromium.org/325503003/)

Reason for revert:
Broke the following layout tests:
- media/track/track-cue-rendering-horizontal.html
- media/track/track-cue-rendering-vertical.html

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=media/track/track-cue-rendering-horizontal.html,media/track/track-cue-rendering-vertical.html


Original issue's description:
> Fix seeking when the start time is non-zero.
> 
> The seek timestamp should always be adjusted by the start time,
> though this manipulation should not be visible to external (web)
> clients.
> 
> Since this is an FFmpeg only problem, I've removed the concept
> of start time from the Demuxer interface in favor of local method
> only for FFmpegDemuxer.  FFmpegDemuxerStream's will now use this
> value to adjust timestamps such that external clients always see
> a zero based timeline.
> 
> Doing so required moving some of our ogg vorbis discard code into
> the FFmpegDemuxerStream, which actually makes it more accurate and
> more narrowly scoped to ogg w/ vorbis instead of all vorbis.
> 
> These changes subtly change how we handle seeking.  Previously we
> would let FFmpeg choose the stream to perform seeking within.  Now
> we always use the stream with the lowest order timestamp.  This
> means we will sometime use the audio stream where previously we
> used the video stream.
> 
> I've extended the tests around non-zero start times to verify the
> new behavior.
> 
> An FFmpeg DEPS roll is required for the new tests to pass:
> 9c12290 Revert "avformat/mp3dec: fix start time in light of initial skip samples"
> 1e661a6 avcodec/vorbisdec: Reset first_frame
> 7d88be4 avformat/oggparsevorbis: Dont attempt to calculate timestamps from gp=0
> 
> BUG=377295
> TEST=new unittests. demo page from bug works.
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277348

TBR=acolwell@chromium.org,dalecurtis@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=377295
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277361

[tkent, 2014-06-16 06:06:07]

Created Revert of Fix seeking when the start time is non-zero.

[commit-bot: I haz the power, 2014-06-16 06:06:52]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tkent@chromium.org/334163002/1

[commit-bot: I haz the power, 2014-06-16 06:08:04]

Change committed as 277361