Get rid of compositing and squashing helper methods on RenderLayerModelObject.

Calling them is basically a layering violation, so it should not be encouraged.

Also made one method of RenderObject private that is already used as such,
and updated RenderLayerClipper::clippingRootForPainting to use an already-available method on RenderLayer that is also used by RenderLayer::enclosingCompositingLayerForPaintInvalidation. This method does almost the same thing as RenderLayerClipper::clippingRootForPainting is intended to do; the latter just throws in an if(transform()).

BUG=384520

Review URL: https://codereview.chromium.org/342203004

git-svn-id: svn://svn.chromium.org/blink/trunk@177055 bbb929c8-8fbe-4397-9dbb-9b2b20218538