GraphicsLayerUpdater should check needsGraphicsLayerUpdate once per CompositedLayerMapping

Previously, all the CompositedLayerMapping member functions checked whether the
CompositedLayerMapping needed a graphics layer update. This CL moves that check
into GraphicsLayerUpdater with the eventual goal of trimming the tree walk to
just those CompositedLayerMappings that actually need to be updated.

Review URL: https://codereview.chromium.org/351813009

git-svn-id: svn://svn.chromium.org/blink/trunk@176853 bbb929c8-8fbe-4397-9dbb-9b2b20218538