Remove invalidations in RenderLayer::removeOnlyThisLayer

When we remove a RenderLayer, it's because of a style change
that should mark the renderer for invalidations anyway. If we
don't, we should fix RenderObject and not rely on RenderLayer.

Review URL: https://codereview.chromium.org/572813003

git-svn-id: svn://svn.chromium.org/blink/trunk@182085 bbb929c8-8fbe-4397-9dbb-9b2b20218538