RenderLayer::scrollParent should return NULL for normal flow layers

We never have a scroll parent if we're normal flow only (since we'll
certainly be dragged along with some other layer). This CL ensures
that RenderLayer::scrollTo returns false in this case.

BUG=309651
R=hartmanng@chromium.org, jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/28833003

git-svn-id: svn://svn.chromium.org/blink/trunk@160126 bbb929c8-8fbe-4397-9dbb-9b2b20218538