RenderLayerScrollableArea::updateResizerStyle shouldn't do work if there's no resizer

In profiling updateStyle on Silk's toggle_drawer test case, we saw some time
being spent updating the style of the resizer, which is wasted work because
there is no resizer. This CL teaches the code not to do work if there's no
resizer.

R=esprehn@chromium.org
BUG=335703

Review URL: https://codereview.chromium.org/182733014

git-svn-id: svn://svn.chromium.org/blink/trunk@168555 bbb929c8-8fbe-4397-9dbb-9b2b20218538