Make [TreatNullAs=NullString] DOMString arguments nullable instead

An IDL method declaration such as

  void f([TreatNullAs=NullString] DOMString x);

can be transformed into

  void f(DOMString? x);

without affecting behavior or generated code, so do that.

Arguments that also have [TreatUndefinedAs=NullString] can't be converted
this way (yet), so this CL leaves them unchanged for now.

Review URL: https://codereview.chromium.org/367663002

git-svn-id: svn://svn.chromium.org/blink/trunk@177322 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r177310

http://src.chromium.org/viewvc/blink?view=revision&revision=177310

BUG=173443
TBR=klemen.forstneric@gmail.com

Review URL: https://codereview.chromium.org/366683002

git-svn-id: svn://svn.chromium.org/blink/trunk@177323 bbb929c8-8fbe-4397-9dbb-9b2b20218538