Delete RenderLayerCompositor::requiresOverhangLayers

This function just returns whether we're the main frame.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/329423002

git-svn-id: svn://svn.chromium.org/blink/trunk@175996 bbb929c8-8fbe-4397-9dbb-9b2b20218538