Remove unnecessary call to containingBlock in RenderLayerModelObject::styleDidChange

We already check parent(). There's no need to check whether we have a containing block.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/436443002

git-svn-id: svn://svn.chromium.org/blink/trunk@179271 bbb929c8-8fbe-4397-9dbb-9b2b20218538