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This CL makes the native window occlusion tracking code register
a global event hook for EVENT_OBJECT_SHOW/HIDE events. When we get one
of those events, check if the window class is either
"MultitaskingViewFrame" (alt tab) or "TaskListThumbnailWnd" (taskbar
preview). If either of these is shown, mark all non minimized root
windows as visible, so that their thumbnail will appear, and things
like YouTube videos will play in the thumbnail. When either of these
is hidden, recalculate occlusion.

The downside of this approach is that we will mark all Chrome root
windows as unoccluded when any app's thumbnail is shown from the
taskbar. I think it's worth doing this to fix the thumbnail regressions.
There can also be a bit of a delay before rendering resumes in the
thumbnails.

Bug: 1137110, 993776
Change-Id: I5528d612822517071dc06904f9093ee5ee9f9f81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503815Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822246}