Replace FINAL and OVERRIDE with their C++11 counterparts in Source/core/dom/custom

Replace |FINAL| with |final| and |OVERRIDE| with |override|
as per allowed C++11 features.

BUG=417463

Review URL: https://codereview.chromium.org/611233002

git-svn-id: svn://svn.chromium.org/blink/trunk@183248 bbb929c8-8fbe-4397-9dbb-9b2b20218538