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The existing code was correcting twice for discarded codec delay
frames.  Once by adjusting the initial timestamp and twice by
discarding those frames before generating the output timestamp.

This change also adds a test helper to AudioFileReader so we can
demux audio files in tests without duplicating code.  As a bonus
it provides some additional coverage to AudioFileReader.

I've also reused several FFmpegCommon methods where possible.

BUG=360961
TEST=Adds opus unit tests. Plus a basic playback test for ogg.
NOTRY=true

Review URL: https://codereview.chromium.org/257563007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266639 0039d316-1c4b-4281-b951-d872f2087c98