Remove WebView::setScrollbarColors

Follow up after blink r159988 and chromium r230163.
Remove unused noop API.

r=jochen@chromium.org

Review URL: https://codereview.chromium.org/31043005

git-svn-id: svn://svn.chromium.org/blink/trunk@160381 bbb929c8-8fbe-4397-9dbb-9b2b20218538