Move 'designMode' attribute from HTMLDocument to Document

Move 'designMode' attribute from HTMLDocument to Document to comply with the
specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/editing.html#making-entire-documents-editable:-the-designmode-idl-attribute

This is consistent with IE11, however, Firefox 28 still has the 'designMode'
attribute on HTMLDocument.

BUG=362955
TEST=fast/dom/Document/designMode-non-html.html

Review URL: https://codereview.chromium.org/236983004

git-svn-id: svn://svn.chromium.org/blink/trunk@171574 bbb929c8-8fbe-4397-9dbb-9b2b20218538