Replace many [TreatReturnedNullStringAs=Null] with nullable DOMString

In IDL files, the attribute declarations

  [TreatReturnedNullStringAs=Null] readonly attribute DOMString X;
  [TreatReturnedNullStringAs=Null, TreatNullAs=NullString] attribute DOMString X;

can now be changed into

  readonly attribute DOMString? X;
  attribute DOMString? X;

without changing behavior or generated code, so do that.

Similarly for methods, the declaration

  [TreatReturnedNullStringAs=Null] DOMString f();

can be changed into

  DOMString? f();

without changing behavior or generated code, so do that too.

Review URL: https://codereview.chromium.org/361883004

git-svn-id: svn://svn.chromium.org/blink/trunk@177302 bbb929c8-8fbe-4397-9dbb-9b2b20218538