Rename RenderLayerBacking to CompositedLayerMapping

The old name may have been reasonable long ago, but for some time
now, it has been quite a misleading misnomer. RenderLayerBacking
is not a composited backing for a RenderLayer. That role is held by
GraphicsLayers that have drawsContent == true. Instead, this class
manages a small cluster of GraphicsLayers and provides the mapping
between RenderLayer paint phases and GraphicsLayers.

Renaming this will not only be clearer for those who are not
intimately familiar with the Blink compositor code, but it will
also help clarify how this class should evolve further.

BUG=233466

Review URL: https://codereview.chromium.org/25737009

git-svn-id: svn://svn.chromium.org/blink/trunk@158905 bbb929c8-8fbe-4397-9dbb-9b2b20218538