Implement Custom Elements 'extends' option.

This enables creating type extensions of elements without a
differentiated prototype, like <section> (whose interface is
HTMLElement) and elements without a distinct prototype, like <ins> and
<del> (whose interface is HTMLModElement.)

This relaxes a lot of prototype chain checks, in line with recent
changes in the spec Working Draft.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/23717043

git-svn-id: svn://svn.chromium.org/blink/trunk@157559 bbb929c8-8fbe-4397-9dbb-9b2b20218538