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This Cl shouldn't have been reverted. See https://crbug.com/784879#c5

aura::WindowOcclusionTracker notifies interested windows
when their occlusion state change.

This will be used to release resources and throttle execution
of occluded tabs.

Time to recompute occlusion on a Samsung Chromebook Pro:
 100 partially occluded windows*: 108us
  50 partially occluded windows*: 50us
  * window 0 at position (x=0;y=0), window 1 at position (x=1;y=1),
    window 2 at position (x=2;y=2), etc.
    Benchmark code:
    https://chromium-review.googlesource.com/c/chromium/src/+/718979

TBR=sadrul@chromium.org

Bug: 738387, 784879
Change-Id: Ie5a8559378223bb78693cd0cd40e4468889e9f79
Reviewed-on: https://chromium-review.googlesource.com/773118Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517083}