More refactoring towards more generic treatment of RenderLayers contributiong to a CompositedLayerMapping.

1. Rename adjustBoundsForSubpixelAccumulation to
computeBoundsOfOwningLayer, which is what it really does.

2. Rename delta to snappedOffsetFromCompositedAncestor, which is what it is. Add offsetFromCompositedAncestor.

3. Remove dependence of updateSquashingLayerGeometry on any
member variables of CompositedLayerMapping.


BUG=372837

Review URL: https://codereview.chromium.org/292893003

git-svn-id: svn://svn.chromium.org/blink/trunk@174418 bbb929c8-8fbe-4397-9dbb-9b2b20218538