Make DocumentType return empty publicId by default

Return empty publicId, systemId by default to be conforming to DOM4 spec:

Unless explicitly given when a doctype is created, its public ID and system ID are the empty string.
http://dom.spec.whatwg.org/#documenttype

This matches Firefox.

BUG=342564

Review URL: https://codereview.chromium.org/167563002

git-svn-id: svn://svn.chromium.org/blink/trunk@167625 bbb929c8-8fbe-4397-9dbb-9b2b20218538