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Previously, we would reposition using the backplane, but with this
change, we introduce a frame around the content window that can be
dragged.

This change also fixes the positioning of the fullscreen toast and
the close button so that they adapt to a resized window.

It also removes some now-useless code for setting the center
position of a gradient quad and fixes a bug in the "bounds contain
children" code where we weren't updating positioning correctly
for children with asymmetric padding.

Bug: 799270
Cq-Include-Trybots: luci.chromium.try:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_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: Ia6d9759b3e8b6fc80bc0d1ae2f0c739cc9cd0f7f
Reviewed-on: https://chromium-review.googlesource.com/967350
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544061}