Simplify computation of scrollParent

Now that we compute the ancestorScrollingLayer during the
CompositingInputsUpdater walk, it's much easier to compute the scrollParent.
This CL also lets us remove the notion of an "inherited scroll parent."

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/350043002

git-svn-id: svn://svn.chromium.org/blink/trunk@176780 bbb929c8-8fbe-4397-9dbb-9b2b20218538