We shouldn't spend 9% of the compositing update computing clippingContainer

This CL moves the computation of the clipping container to the compositing
input update, which can compute it much faster during a pruned downward tree
walk.

This CL makes the compositing update for dragging the drawer in Polymer's calculator
app 9% faster.

BUG=387134

Review URL: https://codereview.chromium.org/354883004

git-svn-id: svn://svn.chromium.org/blink/trunk@176959 bbb929c8-8fbe-4397-9dbb-9b2b20218538