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This patch makes WebContentsViewAura manage its web content's visibility
state based on its current mirror window's occlusion state. To achieve
this, the patch makes the following changes:
 - Adds a new callback in WindowObserver to receive occlusion state
   changes.
 - Creates a subclass of aura::WindowObserver to manage mirror window
   occlusion states for the WebContentsViewAura.
 - Replaces a DCHECK with an early return when an already tracked window
   is requested to be tracked again.
 - Replaces the boolean window property of mirror window with a vector
   list of mirror windows.
 - Updates the unittest and adds a new method to WindowTestApi.

Bug: 901555
Change-Id: I70566c544def6129c7410af84f91e6501a5479d5
Component: Window Mirror View, web contents, Window occlusion tracker
Reviewed-on: https://chromium-review.googlesource.com/c/1313753Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612457}