IDL: Use V8StringResource<WithNullCheck> for nullable DOMString

For methods this means that the two methods

  void f(DOMString? arg)
  void g([TreatNullAs=NullString] DOMString arg)

behave the same. Before this fix, the nullability of f()'s argument was
simply ignored.

For attributes this means that the C++ setter is called with a null string
if a null value is assigned to the attribute.

There are no CG changes since there are neither nullable string arguments
nor writable nullable string attributes in our IDL files. There are such
things in specifications, so supporting this is meaningful.

Review URL: https://codereview.chromium.org/345883004

git-svn-id: svn://svn.chromium.org/blink/trunk@176998 bbb929c8-8fbe-4397-9dbb-9b2b20218538