Use the two Custom Elements node bits to explicitly represent four states.

Recent proposals on
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=20488#c7> change the
relationship between a Custom Element and its registration context. By
explicitly recording whether a custom element is an upgrade candidate,
has a definition, or has been upgraded we can implement this new model
were the custom element won't rely on the registration context after
it gets a definition.

BUG=255725

Review URL: https://chromiumcodereview.appspot.com/19900002

git-svn-id: svn://svn.chromium.org/blink/trunk@154627 bbb929c8-8fbe-4397-9dbb-9b2b20218538