Support getElementById and other methods of ShadowRoot on DocumentFragment

DocumentFragment has querySelector and friends, but doesn't have
getElementById or getElementByClass etc. Because of this the methods were
explicitly added to ShadowRoot which seems kind of silly. There's also
innerHTML on ShadowRoot but not on DocumentFragment.

https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#shadow-root-object

Could we get these methods added to DocumentFragment instead?

 HTMLElement getElementById(in DOMString elementId);
 NodeList getElementsByClassName(in DOMString tagName);
 NodeList getElementsByTagName(in DOMString className);
 NodeList getElementsByTagNameNS(DOMString namespace, DOMString localName);
 attribute DOMString innerHTML;

- Elliott