Refactoring in preparation for Layer Squashing v2.

A prerequisite for Layer Squashing version 2 is that multiple arbitrary
subtrees of RenderLayers need to be able to contribute to the same
GraphicsLayer. This patch creates a simple data structure that
represents information needed for a partial RenderLayer subtree
to paint into a GraphicsLayer.  This allows us to de-couple the
RenderLayer tree from GraphicsLayer tree for more compositing
flexibility.

Subsequent patches (not required for layer squashing) will move the
paintPhase from GraphicsLayer to RenderLayerBacking, and keep track
of a "PaintTask" for each GraphicsLayer within RenderLayerBacking.

BUG=261605

Review URL: https://chromiumcodereview.appspot.com/18808005

git-svn-id: svn://svn.chromium.org/blink/trunk@155466 bbb929c8-8fbe-4397-9dbb-9b2b20218538