Measure usage for DocumentType attributes removed from DOM4

Measure usage for the following DocumentType attributes removed from DOM4:
- entities
- notations
- internalSubset

This will help us determine if we can deprecate them or if they should be
reintroduced in DOM4.

Specification:
http://dom.spec.whatwg.org/#interface-documenttype

R=arv
BUG=272096

Review URL: https://chromiumcodereview.appspot.com/22876008

git-svn-id: svn://svn.chromium.org/blink/trunk@156014 bbb929c8-8fbe-4397-9dbb-9b2b20218538