GraphicsLayerUpdater shouldn't touch CompositedLayerMappings that don't needsGraphicsLayerUpdate

This CL is a warmup for trimming the GraphicsLayerUpdater treewalk to just
those CompositedLayerMappings that have their m_needsGraphicsLayerUpdate bit
set. Most of the work we were doing in this update was already scoped to that
bit. This CL moves the rest of the work inside.

Review URL: https://codereview.chromium.org/331223009

git-svn-id: svn://svn.chromium.org/blink/trunk@176878 bbb929c8-8fbe-4397-9dbb-9b2b20218538