Implemented Document.contentType

http://dom.spec.whatwg.org/#dom-document-contenttype

BUG=279426

Review URL: https://codereview.chromium.org/151653004

git-svn-id: svn://svn.chromium.org/blink/trunk@170646 bbb929c8-8fbe-4397-9dbb-9b2b20218538