createTreeWalker and createNodeIterator should deal with null NodeFilter in Document.

spec: http://dom.spec.whatwg.org/#interface-document
According to spec, optional third parameter(NodeFilter)
of createTreeWalker and createNodeIterator can be null.

So this CL modified behavior as per specified.

TEST=LayoutTests/fast/dom/TreeWalker/TreeWalker-basic.html
    ,LayoutTests/fast/dom/NodeIterator/NodeIterator-basic.html

In addition, with this CL,
Optional arguments to createTreeWalker should be optional (3 passed, null) test
at http://w3c-test.org/dom/nodes/Document-createTreeWalker.html will be passed.

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

Review URL: https://codereview.chromium.org/366223006

git-svn-id: svn://svn.chromium.org/blink/trunk@177560 bbb929c8-8fbe-4397-9dbb-9b2b20218538