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The upcoming VideoRenderer needs a way to know when time starts and
stops moving; this extends the RendererImpl to let the VideoRenderer
know about time changes.

I considered introducing a TimeSourceDependent observer-like interface
for TimeSources, but it ended up being way more complicated. I also
considered a proxy TimeSource, but again it was way more complicated.
The approach here is quite simple and small in complexity.

BUG=439548
TEST=new unittest.

Review URL: https://codereview.chromium.org/1068593003

Cr-Commit-Position: refs/heads/master@{#324172}