Fix RenderBlock::nodeAtPoint() for scrollbars of hidden and pointer-events:none elements

Make sure that the RenderBlock is visible to the hit test request before
testing whether the test point is in an overflow control.

This is similar to the code a few lines down that performs hit testing
for the background.

BUG=313908

Review URL: https://codereview.chromium.org/68243009

git-svn-id: svn://svn.chromium.org/blink/trunk@164436 bbb929c8-8fbe-4397-9dbb-9b2b20218538