WebmMuxer: stop inputting negative timestamps to mkvmuxer.

WebmMuxer feeds signed timestamps into a mkvmuxer
method taking only unsigned timestamps.
This turned into problems muxing data for the case the
MediaRecorder was immediately paused on startup, and
WebmMuxer would try to feed negative timestamps to
mkvmuxer.

The change makes sure we do not insert negative timestamps.

Bug: 1045381, 1057810
Change-Id: I3ea00bb3f4d372e9f6ad4eeba484b0cb8a9582fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095071
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/master@{#748382}