Simplify RenderLayerCompositor::rootShouldAlwaysComposite

This function was written in a confusing way.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/324513003

git-svn-id: svn://svn.chromium.org/blink/trunk@175691 bbb929c8-8fbe-4397-9dbb-9b2b20218538