Update DOM classes to use OVERRIDE / FINAL when needed

Update DOM classes to use OVERRIDE / FINAL when needed. Those are useful for
error checking or performance when the compiler supports it.

R=esprehn, abarth
BUG=None

Review URL: https://codereview.chromium.org/126633004

git-svn-id: svn://svn.chromium.org/blink/trunk@164668 bbb929c8-8fbe-4397-9dbb-9b2b20218538