Switch GetWallClockTime to using vectors for input and output.
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}