Clean up virtual functions in css/

Clean up virtual functions in css/ by:
- Making classes final when possible
- Using 'override' when appropriate
- Making isXXX() virtual functions private on XXX classes

I also added a few nullptr usages.

Based on:
http://trac.webkit.org/changeset/175391

Review URL: https://codereview.chromium.org/719833002

git-svn-id: svn://svn.chromium.org/blink/trunk@185243 bbb929c8-8fbe-4397-9dbb-9b2b20218538