Move 'embeds' / 'plugins' attributes from HTMLDocument to Document

Move 'embeds' / 'plugins' attributes from HTMLDocument to Document to comply
with the specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#dom-document-embeds

This behavior is consistent with IE11. However, Firefox 28 still exposes those
attributes on HTMLDocument.

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

Review URL: https://codereview.chromium.org/237233002

git-svn-id: svn://svn.chromium.org/blink/trunk@171682 bbb929c8-8fbe-4397-9dbb-9b2b20218538