Clean up the audio timestamp and duration code.
Clean up the audio timestamp and duration code.

While debugging this code I found it to be a tangled web of logic.  The new version is much simpler:
  - Always calculate a duration (this makes sense anyway)
  - Use the estimated timestamp when one isn't available
  - Reset the estimated timestamp when a valid timestamp is available

BUG=26036
TEST=everything still plays in sync

Review URL: http://codereview.chromium.org/1695009

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@45339 0039d316-1c4b-4281-b951-d872f2087c98

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