Fix LayoutTest gesture-tap-frame-scrollbar.html

This test was dependant on the event handling behavior of scrollbars,
which varies across different OSes.  In particular, on most OSes
(including MacOS 10.6) we were seeing events pass through frame scrollbars
to be received by the page underneath.  On MacOS 10.7+ the scrollbars
appear to swallow the events so the page never sees them.

What we really care about in this test is blink behavior, so switch
to using mock scrollbars to make the test behavior consistent.

BUG=414034

Review URL: https://codereview.chromium.org/547293005

git-svn-id: svn://svn.chromium.org/blink/trunk@182188 bbb929c8-8fbe-4397-9dbb-9b2b20218538