Fix RenderLayer::scrollsWithRespectTo().

scrollsWithRespectTo() currently tries to match up
containing blocks by keeping track of RenderObjects.
However, when it goes to check if a containing block
is a scrolling div, it actually checks the enclosing
RenderLayer. It doesn't make sense to do this since
RenderLayers and RenderObjects don't line up
one-to-one.

Since this function only cares about scrollability,
and the RenderLayer is the object that handles an
elements overflow-scrollability, it makes more
sense to keep track of containing blocks by storing
the enclosingLayer() instead of the RenderObject
itself.

BUG=384642

Review URL: https://codereview.chromium.org/343623003

git-svn-id: svn://svn.chromium.org/blink/trunk@176452 bbb929c8-8fbe-4397-9dbb-9b2b20218538