Make compositing update 15% faster on Silk toggle_drawer page set

Before this CL, we were spending 28% of the time during compositing update in
RenderLayerClipper::backgroundClipRect. However, the primary caller of that
function wasn't actually interested in the clip rect. That caller just wants to
know whether a clip exists at all.

This CL changes RenderLayerCompositor::clippedByAncestor to just compute
whether the layer is clipped at all (rather than what the clip rect actually
is). After this CL, we only spend 13% of the compositing update profile in
RenderLayerClipper::backgroundClipRect.

Review URL: https://codereview.chromium.org/177533021

git-svn-id: svn://svn.chromium.org/blink/trunk@167869 bbb929c8-8fbe-4397-9dbb-9b2b20218538