Remove updateRoot from RenderLayerCompositor::updateCompositingLayers()

This value is actually not meaningful after deferred compositing.
If we want to re-introduce support for this, it will need more hefty
restructuring anyway, so it's better to simplify the code for now.

Review URL: https://codereview.chromium.org/55243008

git-svn-id: svn://svn.chromium.org/blink/trunk@161220 bbb929c8-8fbe-4397-9dbb-9b2b20218538