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WebmMuxer sets WebM's DefaultDuration to the expected delta between
frames (derived from the frame rate), but that messes up (some) players
due to frames arriving at a different rate. This is a legimimate case
if, e.g.  the encoded data stream is paused.

This CL essentially avoids setting this parameter so that players
can just look at each encoded frame timestamp instead.

[1] http://www.webmproject.org/docs/container/#track

BUG=606000

Review-Url: https://codereview.chromium.org/2617143003
Cr-Commit-Position: refs/heads/master@{#442081}