Revert "Revert "Fix border-radius clip with composited scrolling.""

This relands r173970. It was not the cause of the test failure.

BUG=372973
TBR=hartmanng@chromium.org

Review URL: https://codereview.chromium.org/287733002

git-svn-id: svn://svn.chromium.org/blink/trunk@173988 bbb929c8-8fbe-4397-9dbb-9b2b20218538