Remove unused second parameter to scrollContentsFastPath

No one uses this value. We can remove it.

R=skobes@chromium.org

Review URL: https://codereview.chromium.org/480623002

git-svn-id: svn://svn.chromium.org/blink/trunk@180395 bbb929c8-8fbe-4397-9dbb-9b2b20218538