Capture Mode: Restrict capture region edge drag resizing

This CL ensures that when dragging the selected region by an edge
affordance, that the capture region is resized only in the expected
direction.

Test: manual, updated test
Change-Id: Icd2691592deb180db55b84f8e394b2e8935a3858
Fixed: 1140778
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490866


Commit-Queue: Richard Chui <richui@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820054}