RenderLayerCompositor::calculateCompositedBounds arguments are redundant

All the callers pass the same RenderLayer for both arguments.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/226313004

git-svn-id: svn://svn.chromium.org/blink/trunk@170904 bbb929c8-8fbe-4397-9dbb-9b2b20218538