Document.body should check interface type and be nullable

SPEC: http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#document

This CL enabled an interface type checking on setter of Document.body
and a nullable type to body attribute as per specification.

In addition, with this change, a failed test case will be passed in
http://w3c-test.org/html/dom/documents/dom-tree-accessors/document.body-setter-01.html

TEST=LayoutTests/fast/dom/setter-type-enforcement.html

Behavior in other browsers:
*)FF: PASS
*)IE: FAIL

Review URL: https://codereview.chromium.org/392523002

git-svn-id: svn://svn.chromium.org/blink/trunk@178039 bbb929c8-8fbe-4397-9dbb-9b2b20218538