Make Comment constructable

http://dom.spec.whatwg.org/#comment

This allows us to do `new Comment('abc')` instead of
`document.createComment('abc')`.

BUG=238233

Review URL: https://chromiumcodereview.appspot.com/14931009

git-svn-id: svn://svn.chromium.org/blink/trunk@149821 bbb929c8-8fbe-4397-9dbb-9b2b20218538
7 files changed