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With this CL, I change how the visibility of modal dialogs and the
related visibility of 2d browsing scene elements are managed.

I have also added two elements in the ancestor chain for the
audio permission prompt: a shadow and a full screen hit target
(similar to the exit prompt backplane).

Bug: 768512
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ia5dcbe865fb8d92e90538039d9dfaa4679573dac
Reviewed-on: https://chromium-review.googlesource.com/774908
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Yash Malik <ymalik@chromium.org>
Reviewed-by: Biao She <bshe@chromium.org>
Reviewed-by: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517298}