Move the policy for processing Custom Elements into DocumentInit.

This does not change any behavior. But when documents share a Custom
Element registration context, DocumentInit will be the vector that
conveys the registration context to the new document.

TL;DR Soon, trojan:wooden horse :: registration context:DocumentInit.

BUG=247219

Review URL: https://chromiumcodereview.appspot.com/20106005

git-svn-id: svn://svn.chromium.org/blink/trunk@154898 bbb929c8-8fbe-4397-9dbb-9b2b20218538