Remove useSlowRepaints

We don't have a concept of slow repaints anymore.

R=eseidel@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/445213003

git-svn-id: svn://svn.chromium.org/blink/trunk@179684 bbb929c8-8fbe-4397-9dbb-9b2b20218538