Fix pixel snapping of stacked sub-pixel composited layers

Correcting placement and clipping of sub-pixel offset
composited layers by properly adding the accumulated sub-
pixel offset from composited parents.

BUG=347521

Review URL: https://codereview.chromium.org/204053002

git-svn-id: svn://svn.chromium.org/blink/trunk@169495 bbb929c8-8fbe-4397-9dbb-9b2b20218538