Fix pixel snapping of clipLayers and their descendants

Hopefully the last in a line of compositing sub-pixel
patches. This correctly pixel snaps the clipping layer
used by composited layers with overflow: clip and
accumulates it into their children.

BUG=347521

Review URL: https://codereview.chromium.org/205213006

git-svn-id: svn://svn.chromium.org/blink/trunk@169624 bbb929c8-8fbe-4397-9dbb-9b2b20218538