Issue repaints before and after changing the squashing assignment of a RenderLayer.

Also, do not include squashed layers when computing repaint
rects including descendants. Squashed layers paint separately
with a different origin.

BUG=370664

Review URL: https://codereview.chromium.org/309093002

git-svn-id: svn://svn.chromium.org/blink/trunk@175417 bbb929c8-8fbe-4397-9dbb-9b2b20218538