Use nanosecond timestamps when calling into libwebm from media::WebmMuxer.
Use nanosecond timestamps when calling into libwebm from media::WebmMuxer.

While using this class to generate sample files, it became apparent that
the timestamps in the resulting WebM files were incorrect.  The reason
is because the code was providing millisecond resolution timestamps, but
the libwebm API was expecting nanosecond resolution timestamps instead.
libwebm will translate these timestamps into millisecond resolution when
writing out the file data.

R=mcasas@chromium.org

Review URL: https://codereview.chromium.org/1317563006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347198}

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