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This CL enables throttling of child synchronization events. In a child-
initiated synchronization, the child's frame will likely activate
before the dependent parent's CompositorFrame arrives. Upon activation,
the child will see that the new surface does not cause any damage to the
display (because it's not embedded by any parent yet) and calls the
draw callback immediately, thereby sending back a DidReceiveCompositorFrameAck
to the client immediately.

This, in turn, results in the client sending another CompositorFrame to Viz,
and another and so on, letting the child get further and further ahead of the
parent. This results in a lot of wasted work because the child's surface will
not be shown on screen until the parent embeds it and so multiple child
initiated synchronization events are pointless.

This CL throttles DidReceiveCompositorFrameAck of the child on child-initiated
synchronization events until the parent references the child.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I62416d87f8d871b32f4caad565cff515a9c55445
Bug: 672962, 879207
Reviewed-on: https://chromium-review.googlesource.com/1236931
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596300}