Delete useless code in RenderLayer::calculateLayerBounds

This code can't do anything because there's no way to get behind the document
element. It's always promoted to be a stacking context.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/223903004

git-svn-id: svn://svn.chromium.org/blink/trunk@170798 bbb929c8-8fbe-4397-9dbb-9b2b20218538