Implement Custom Elements class-side inheritance.

DOM elements and ES6 do something called "class-side inheritance"
where not only does an instance of subtype B's prototype point to base
type A's prototype, the B constructor's prototype is A's
constructor. This means that B.FOO works where FOO is a static
operation or const attribute (in the WebIDL sense) of A.

This is described in this proposal:
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=22920>

BUG=271466

Review URL: https://chromiumcodereview.appspot.com/23386003

git-svn-id: svn://svn.chromium.org/blink/trunk@156737 bbb929c8-8fbe-4397-9dbb-9b2b20218538