Remove RenderLayerCompositor::compositingLayersNeedRebuild()

No one outside this class should read this private state. This getter is also
misleading because we often compute this dirty bit during the compositing
update (e.g., when we compute the compositing requirements).

Review URL: https://codereview.chromium.org/230243003

git-svn-id: svn://svn.chromium.org/blink/trunk@171149 bbb929c8-8fbe-4397-9dbb-9b2b20218538