Add 'exposed' objects and embeds to a document's named properties.

The spec tells us that exposed <embed> and <object> are to be included
in a document's "supported property names"

  http://www.whatwg.org/specs/web-apps/current-work/#dom-document-nameditem
  http://www.whatwg.org/specs/web-apps/current-work/#exposed

(along with various other side conditions.) 

crbug.com/313292 retired a partial notion of "exposed"ness, this
provides an implementation close to the spec's definition.

R=
BUG=316988

Review URL: https://codereview.chromium.org/75273004

git-svn-id: svn://svn.chromium.org/blink/trunk@163287 bbb929c8-8fbe-4397-9dbb-9b2b20218538