snap-group: Overriding `View::GetCursor` in `SplitViewDividerView`

Demo: http://b/331240308#comment2
Fixed: b/331240308
Test: Added unit test + manually
Change-Id: I610fc0dc64577d670da6b8a3683de8ca8bd852c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406673


Commit-Queue: Michele Fan <michelefan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280189}