[media_preview] Add tests for VideoStreamCoordinator metrics

This covers PixelHeight, ExpectedFPS, ActualFPS,
RenderedPercent.

Fixed: b:329663133, b:329664065, b:329664520, b:330394977
Change-Id: Ida86331e0959718218e5526d1e7a31de42b1684b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401460


Reviewed-by: Ahmed Moussa <ahmedmoussa@google.com>
Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282717}