When performing a fast-path scroll, issue repaints for viewport-constrained layers
relative to their repaint container, not their enclosing composited ancestor.

The latter is wrong for squashing.
BUG=370664

Review URL: https://codereview.chromium.org/318793002

git-svn-id: svn://svn.chromium.org/blink/trunk@175534 bbb929c8-8fbe-4397-9dbb-9b2b20218538