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Input throttling with surface synchronization didn't always work well.

Occasionally a huge number of input events from Ash would be processed
between the time a resize request is first initiated and the time the
surface synchronization event began. This caused a lot of unnecessary
jank. Traces showed that the browser UI process was busy doing a lot of input
processing work but the resize in the renderer consumed very little
time so there was definitely an opportunity to improve input throttling.

This CL takes a different (simpler/better) approach. As soon as a
synchronization event is detected in DelegatedFrameHost (we know we
will produce a UI CompositorFrame that will synchronize with the
renderer), we throttle input events until a browser UI commit
(OnChildResizing guarantees an upcoming commit).

Bug: 672962
Change-Id: I75c8328bb846eff2817a805e63a6367ebb0714a2
Reviewed-on: https://chromium-review.googlesource.com/822658
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523788}