Move subpixelAccumulation from CompositedLayerMapping to RenderLayer.

It is a property specific to a RenderLayer, so this makes more sense. Also,
this makes it possible to properly add in subpixel accumulation when
computing repainting for squashed layers.

BUG=358261

Review URL: https://codereview.chromium.org/222023007

git-svn-id: svn://svn.chromium.org/blink/trunk@170721 bbb929c8-8fbe-4397-9dbb-9b2b20218538