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* A window frame should have rounded corners only in normal state,
(floating state). All other state won't need them so can be
opaque window.

* Don't apply this logic if the opacity is explicitly specified (not kInferred)

* Old code removed the rounded corner only for maximized/fullscreen.
 Instead, this CL checks if it's normal.

* Consolidated the occlusion region update code for transparent window
 to WindowState.

* Update DCHECK in Window::SetOpaqueRegionsForOcclusion to allow resetting.

Bug: 1022544
Test: covered by unit tests. Also tested on the device. OOBE, login screen,
   arc windows works as intended.

Change-Id: Ia8e067945732b81294c3e6733121d433e3087f04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2059089
Commit-Queue: Mitsuru Oshima (OOO) <oshima@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743038}