Merge to XFA: Use override in more classes in core/

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1292613003 .

(cherry picked from commit c2c3f7b5f0396409451a9d344f35ec1929a76e9f)

Review URL: https://codereview.chromium.org/1296043002 .
42 files changed