Remove duplicated invalidation

The code to generate an invalidation for StyleDifferenceRepaint and outline
decrease in RenderLayerModelObject::styleWillChange is duplicated with the
one in RenderObject::styleWillChange. This meant that we would generate 2
identical invalidations for each repaint-cue or outline decrease.

Review URL: https://codereview.chromium.org/217973002

git-svn-id: svn://svn.chromium.org/blink/trunk@170591 bbb929c8-8fbe-4397-9dbb-9b2b20218538