Don't repaint squashed, fixed position layers during scrolling

There's no need to repaint fixed position layers that are squashed. Necessarily
they must be squashed onto other fixed position layers, which means they don't
move during scrolling.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/478823002

git-svn-id: svn://svn.chromium.org/blink/trunk@180410 bbb929c8-8fbe-4397-9dbb-9b2b20218538