Fix OutlineValue::visuallyEqual() to check offset

BUG=399798
TEST=fast/repaint/outline-change-offset.html
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/439013002

git-svn-id: svn://svn.chromium.org/blink/trunk@179505 bbb929c8-8fbe-4397-9dbb-9b2b20218538