[Slimming Paint] Track clip renderers

This patch updates the clip recorder and clip items to track a
RenderLayerModelObject (a subclass of RenderObject) so invalidations
are properly handled in the update algorithm just like regular
DisplayItems. Clipper callsites have been updated accordingly.

With this patch, the ASSERT in ViewDisplayList::findDisplayItem can
now be removed.

TEST=ViewDisplayListTest_UpdateClip

Review URL: https://codereview.chromium.org/719353004

git-svn-id: svn://svn.chromium.org/blink/trunk@185341 bbb929c8-8fbe-4397-9dbb-9b2b20218538