Don't update the clock if we don't have a reliable timestamp available.
Don't update the clock if we don't have a reliable timestamp available.

Turns out vorbis packets only have an accurate timestamp every second or so while the rest of the packets have a timestamp of zero.  Problem was we were overwriting |last_fill_buffer_time_| with zeros and then updating the clock, effectively causing a ton of drift.

Review URL: http://codereview.chromium.org/115593

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@16561 0039d316-1c4b-4281-b951-d872f2087c98

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