RenderLayer::ancestorScrollingLayer() is slow

When dragging the drawer on the calculator, computing the
RenderLayer::ancestorScrollingLayer() repeatedly is 7.8% of the profile. This
CL moves the work of computing the ancestorScrollingLayer into the
CompositingInputsUpdater, where we can compute it more efficiently because
we're walking down the tree.

R=esprehn@chromium.org
BUG=387134

Review URL: https://codereview.chromium.org/330313004

git-svn-id: svn://svn.chromium.org/blink/trunk@176678 bbb929c8-8fbe-4397-9dbb-9b2b20218538