capture_mode: Position capture button and add an ink drop.

Change the capture session widgets to be POPUP. If they are window,
they need to be clicked first to gain capture. For example, press and
drag of capture button. Then to close, you have to first click the bar
and then click the exit button. This happens before this patch, but it
is more noticeable with the inkdrop visuals.

Change ShouldCaptureLabelEvent so that it can get events directed at it
that have no location (ENTER, CAPTURELOST, EXIT).

The capture button is now centered in the region if it can be with a
minimum amount of padding on both sides. Otherwise it will go on either
slightly above or below the region, based on its proximity to the root
bottom edge.

Test: ash_unittests CaptureModeTest.CaptureRegionCaptureButtonLocation
Bug: 1131211
Change-Id: I732a1871120ffa9b9eee4fbadd5361fc9aec0ac0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427865
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815883}