Inline RenderLayerCompositor::calculateCompositedBounds

This function didn't serve a purpose because the branch it contained was never
taken.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/229033005

git-svn-id: svn://svn.chromium.org/blink/trunk@171095 bbb929c8-8fbe-4397-9dbb-9b2b20218538