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On Windows, check if the WindowTreeHost of the window being dropped on
is occluded, and if so, don't allow the drop.

This also enables occlusion calculation during window moving, if the
window being moved is a root window. And it ignores the dragged window
when determining if a window under it is occluded.

Bug: 820998
Change-Id: I8fd95ec3f9c9d1b40a848c697cce91125eff21c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879606
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835713}