Eliminate hit test in handleMouseFocus()

The hit test was to check if the mouse was over a scrollbar, we already have
this information when targeting touch events so plumb that through and reuse.

BUG=381728

Review URL: https://codereview.chromium.org/640853006

git-svn-id: svn://svn.chromium.org/blink/trunk@184109 bbb929c8-8fbe-4397-9dbb-9b2b20218538