Don't assert wrapping Custom Element type extensions of HTMLElements.

Some HTML elements such as SECTION don't have a distinct DOM
interface; they just use HTMLElement. When one of these is used as a
Custom Elements type extension, Custom Elements wrapping machinery
needs to produce a "direct" wrapper, just like when they're wrapped by
usual means.

This defect was found by the fuzzing described in
<http://crbug.com/234509#c40>.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/22057003

git-svn-id: svn://svn.chromium.org/blink/trunk@155542 bbb929c8-8fbe-4397-9dbb-9b2b20218538