Fix corrupted audio and video at playback start in ogg containers.

Fix corrupted audio and video at playback start in ogg containers.

When demuxing ogg containers, FFmpeg will seek to the first packet in the
chosen stream that matches the seek criteria.  However, it does not advance
or rewind other streams in the container to match this timeline.

It's an open question whether FFmpeg is doing the wrong thing here:
http://lists.ffmpeg.org/pipermail/ffmpeg-devel/2014-June/159212.html

To keep things working in the meantime negative audio start times will
be clipped to zero when being considered for seeking.

Additional fixes:
- Fixes non-audio timestamps being incorrectly offset when negative
timestamps are discarded from the audio stream.

BUG=387996
TEST=new unittest, extended existing tests.

Review URL: https://codereview.chromium.org/353563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@280720 0039d316-1c4b-4281-b951-d872f2087c98