Get rid of paint invalidation code in RenderObject*::styleWillChange for cases that require layout.

Instead do it only when it is really needed, in particular
in RenderLayer::removeOnlyThisLayer (which is needed
because repaint-after-layout doesn't understand destructive
updates to the layer tree).

Also gets rid of a compositing assert that is no longer
needed as of crrev.com/371913005.

BUG=391036

Review URL: https://codereview.chromium.org/373843002

git-svn-id: svn://svn.chromium.org/blink/trunk@177697 bbb929c8-8fbe-4397-9dbb-9b2b20218538