Fix middle mouse drag cursor without regressing menu-over-link cursor

A previous change
https://src.chromium.org/viewvc/chrome?revision=224273&view=revision to fix
the mouse cursor not being set properly over menus caused a regression in
middle-mouse dragging. Because the root window doesn't get focus until after
the mouse down, the middle drag cursor doesn't get set on mouse down as it
should.

So, instead of using RootWindow focus, check that there's not another top
level window (like the menu) above the root window that's asking to set the
cursor instead.

R=ben@chromium.org
BUG=292379,299286

Review URL: https://codereview.chromium.org/25076004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225861 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225861