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Fullscreen, window, region capture mode now all require change mouse
cursor. This CL put mouse cursor setting logic in a central place and
auto decide which cursor to use based on the current event location.
It unified the cursor behavior on the capture bar and button, and also
fixed a few bugs in the original implementation, like the mouse cursor
can be disappear after clamshell->tablet->clamshell transition.

Bug: 1142937
Change-Id: I982e3be0fd9c513f108a8dd7cfe4e8aea60905e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537939
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828564}