Associate Custom Element callbacks with the right context.

In preparation for sharing a Custom Element registration context
between multiple documents, associate the Custom Element callbacks
with the active script context and not the specific document that
received the call to document.register (although the generated
constructor uses that document to own elements it produces.)

BUG=247219

Review URL: https://chromiumcodereview.appspot.com/20103004

git-svn-id: svn://svn.chromium.org/blink/trunk@154904 bbb929c8-8fbe-4397-9dbb-9b2b20218538