Add support for DocumentFragment.getElementById()

Add support for DocumentFragment.getElementById() as specified in the DOM
specification:
http://dom.spec.whatwg.org/#interface-nonelementparentnode

The patch moves the SVGSVGElement::getElementById() implementation up to
ContainerNode so that it can be shared with DocumentFragment.

Firefox 28 already supports it, IE11 does not.

R=arv@chromium.org, abarth@chromium.org
BUG=366927
TEST=fast/dom/DocumentFragment/get-element-by-id.html

Review URL: https://codereview.chromium.org/251633002

git-svn-id: svn://svn.chromium.org/blink/trunk@172796 bbb929c8-8fbe-4397-9dbb-9b2b20218538