Factor GraphicsLayerUpdater out of RenderLayerCompositor

This CL moves some code relating to walking the RenderLayer tree and updating
GraphicsLayers out of RenderLayerCompositor. This code didn't interact with any
of the state in RenderLayerCompositor, so it's easy to untangle.

I've also taken the opportunity to remove a pretty gross hack using static
variables to communicate data across the recursive tree walk. Now we just use
normal member variables in the stack object that manages the tree walk.

This CL shouldn't change any behavior.

Review URL: https://codereview.chromium.org/185293002

git-svn-id: svn://svn.chromium.org/blink/trunk@168244 bbb929c8-8fbe-4397-9dbb-9b2b20218538