Compositor touch hit rects report unnecessary rects for non-composited scrolling layers

A non-composited scrolling layer will get projected to the same compositing
layer as its bounding box so to make its visualization easier don't report
m_scrollableArea->overflowRect().

BUG=337844

Review URL: https://codereview.chromium.org/148423004

git-svn-id: svn://svn.chromium.org/blink/trunk@166084 bbb929c8-8fbe-4397-9dbb-9b2b20218538