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I removed the optimizations in SyncPointClientImpl where it'd skip
generating sync tokens if there was no existing release sync token.
That caused a 5% cpu time regression in the video benchmarks.

I also added some tests to verify that sync tokens are being generated
at the right places.

R=jbauman
BUG=750872

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If739e3fb6705cf8b0739fc029ed3bd75dc43eabf
Reviewed-on: https://chromium-review.googlesource.com/596424Reviewed-by: John Bauman <jbauman@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491560}