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Ensures that all converted timestamps will be monotonically increasing
within a single call to GetWallClockTime.

Performance seems unchanged (unit tests take the same amount of time).

As part of this change, also does the following:
- VideoRendererImpl unit test switched to WallClockTimeSource to avoid
having to reimplement the vector conversion routine.

BUG=485042
TEST=new unittest.

Review URL: https://codereview.chromium.org/1136513004

Cr-Commit-Position: refs/heads/master@{#329363}