Document.adoptNode should not throw an exception
for DocumentType argument

According to spec, Document.adoptNode only throws exception on document.

spec: http://dom.spec.whatwg.org/#dom-document-adoptnode
  *)If node is a document, throw a "NotSupportedError" exception.

So this CL modified behavior as per specified.

TEST=LayoutTests/fast/dom/Document/adopt-node-with-doctype.html

In addition, with this CL, http://w3c-test.org/dom/nodes/Document-adoptNode.html
will be passed.

Behavior in other browsers:
  *)FF: pass
  *)IE: fail

Review URL: https://codereview.chromium.org/364173005

git-svn-id: svn://svn.chromium.org/blink/trunk@177501 bbb929c8-8fbe-4397-9dbb-9b2b20218538