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This CL implements an observer pattern for activated child surface,
where each surface will be able to observe activation events
happening in a subset of FrameSinkIds and update SurfaceReferences
accordingly. This is needed since we will no longer refer to the fallback
but instead will use GetLatestInFlightSurface to acquire references from
the SurfaceRanges specified in CompositorFrameMetaData.

Bug: 857575
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8ee1a9633273e116c5e7b1ae840dcc8db7a0407a
Reviewed-on: https://chromium-review.googlesource.com/1157404
Commit-Queue: Andre Kaba <akaba@google.com>
Reviewed-by: Saman Sami <samans@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582773}