Share the Custom Element registration context with the template document.

This means the template document shares the set of Custom Element
definitions with the main document, so custom element instances in the
template document go through upgrade and have the prototypes and API
that authors expected.

BUG=247219

Review URL: https://chromiumcodereview.appspot.com/20121004

git-svn-id: svn://svn.chromium.org/blink/trunk@154912 bbb929c8-8fbe-4397-9dbb-9b2b20218538