Fix bug in RenderLayer::scrollsWithRespectTo.

If one layer is a child of the other, but neither scrolls, in some cases
it would return incorrectly that one scrolled w.r.t. the other. In
particular this would be true if one of the layers was the child of an
overflow scroll div.

BUG=368410

Review URL: https://codereview.chromium.org/268813005

git-svn-id: svn://svn.chromium.org/blink/trunk@173265 bbb929c8-8fbe-4397-9dbb-9b2b20218538