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Backplane of the hosted dialogs are moved to align
with main content. As a result the dialog itself
will be in the center of the web content.

BUG=808560

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I6addedbfef905c51e11d5a6a5c95f889775a2078
Reviewed-on: https://chromium-review.googlesource.com/953247Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541613}