Add toHTMLDocument() convenience methods that take a reference

Add toHTMLDocument() convenience methods that take a reference as several
call sites have a Document reference, not a pointer. This avoids having
to convert the reference into a pointer.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23591020

git-svn-id: svn://svn.chromium.org/blink/trunk@157046 bbb929c8-8fbe-4397-9dbb-9b2b20218538