Don't add subpixel accumulations for squashed layers when invalidating them.

Also clean up the code related to squashing in CompositedLayerMapping a little bit.

BUG=417994

Review URL: https://codereview.chromium.org/659493005

git-svn-id: svn://svn.chromium.org/blink/trunk@183703 bbb929c8-8fbe-4397-9dbb-9b2b20218538