Use scrollContentsSlowPath when scrolling with background-attachment: fixed

This CL causes us to bail out of the fast scrolling path if we have slow
repainting objects (i.e., background-attachment: fixed). We've done this for a
long time, by my recent CL removed this behavior and we appear to lack test
coverage for it.

R=vollick@chromium.org
BUG=402747

Review URL: https://codereview.chromium.org/474683002

git-svn-id: svn://svn.chromium.org/blink/trunk@180223 bbb929c8-8fbe-4397-9dbb-9b2b20218538