Fix Custom Elements use-after-free when deleting an upgrade candidate.

The registration context's upgrade candidate map wasn't being cleaned
up properly when a type extension upgrade candidate was deleted.

This also makes removing an element's type extension the last thing
that happens as custom element processing winds down in its twilight
moments.

BUG=255725

Review URL: https://chromiumcodereview.appspot.com/19671007

git-svn-id: svn://svn.chromium.org/blink/trunk@154470 bbb929c8-8fbe-4397-9dbb-9b2b20218538