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This adds unittests for the changes made in
crrev.com/a6a3f1e8

This requires a small refactoring to allow unittests to fake the
current time for the WebrtcFrameSchedulerSimple class.

Bug: 773894
Change-Id: I00e1204b1eab446e18e5d8796770dbd64623abfc
Reviewed-on: https://chromium-review.googlesource.com/729565
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Zijie He <zijiehe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512914}