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This implements native window occlusion tracking on windows, under flag/experiment control. It uses WinEvent event hooks and EnumWindows to track and calculate occlusion.

It is not yet hooked up to the code that treats occluded windows differently.

Bug: 813093
Change-Id: Ic9b473b8cad38e7b1653566f28917164bc92e21c
Reviewed-on: https://chromium-review.googlesource.com/c/1140752
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603619}