Rebuild composited layer tree when a layer's need for a backing changes

This makes RenderLayerCompositor::allocateOrClearCompositedLayerMapping
recognize when an existing composited layer's need for a backing changes
without the layer's need for compositing changing.

BUG=256200

Review URL: https://codereview.chromium.org/26515003

git-svn-id: svn://svn.chromium.org/blink/trunk@161263 bbb929c8-8fbe-4397-9dbb-9b2b20218538