Have CharacterData constructor take a Document reference in argument

Have CharacterData constructor take a Document reference in argument instead
of a pointer as it can never be NULL. CharacterData subclasses (Comment,
Text), have been updated to use references as well.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23754005

git-svn-id: svn://svn.chromium.org/blink/trunk@157373 bbb929c8-8fbe-4397-9dbb-9b2b20218538