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In window capture mode, the user should be able to interactively select
a window to capture. when the to-be-captured window's bounds or state is
changed, we should update the capture region to reflect the change as
well.

Other changes that is worth mentioning in this CL:
- Window stacking order or activation order will not be changed by the
capture window selection change
- If current selected is closed/minimized or becomes invisible, we
automatically select another window under mouse/touch event location.
and if there is no such window, do nothing.
- If another window is activated when a window is selected, if the
activated window is under mouse/touch event location, it will be focused
instead.
- When a mouse moved over a selectable window, change its cursor to
either capture icon or recording icon

Bug: 1127526
Change-Id: Icfdbed419dadfe0301b60da693f01010f4a2da2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421090Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812976}