Remove unused clip rect in fast scrolling path

No one uses this parameter. We can remove it.

R=skobes@chromium.org

Review URL: https://codereview.chromium.org/448943003

git-svn-id: svn://svn.chromium.org/blink/trunk@179710 bbb929c8-8fbe-4397-9dbb-9b2b20218538