Don't share the Custom Element registration context with templates.

Elements in templates are meant to be inert. (Scripts don't run, for
example.) So that Custom Elements aren't "live" (upgraded, running
callbacks) in templates, stop sharing the main document's registration
context with the template document.

Authors that want Custom Element definitions to be active in templates
can register definitions with the template document directly.

This is described here:
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=23839>

BUG=234509

Review URL: https://codereview.chromium.org/72773003

git-svn-id: svn://svn.chromium.org/blink/trunk@162301 bbb929c8-8fbe-4397-9dbb-9b2b20218538