Move Custom Element:definition mapping out of the registration context.

So that Custom Elements may outlive their registration context, it is
necessary to move per-instance state out of the registration context.

BUG=255725

Review URL: https://chromiumcodereview.appspot.com/19382006

git-svn-id: svn://svn.chromium.org/blink/trunk@154725 bbb929c8-8fbe-4397-9dbb-9b2b20218538