RenderLayer::clipParent() should avoid calling clippedByAncestor unless needed

Calling clippedByAncestor is not free, we should avoid calling it unless
the compositingReasons() say we need to.

R=ojan@chromium.org,vollick@chromium.org,abarth@chromium.org

Review URL: https://codereview.chromium.org/183263003

git-svn-id: svn://svn.chromium.org/blink/trunk@168234 bbb929c8-8fbe-4397-9dbb-9b2b20218538