Bug 1149987 - Part 2: Make ErrorResult unassignable; r=bzbarsky

This object is not copyable, so it should probably not be
assignable either, especially since the compiler generated
copy constructor can potentially cause UAF issues on the
mMessage member, for example.

dom/bindings/ErrorResult.h

@@ -179,6 +179,7 @@ private:
   // Not to be implemented, to make sure people always pass this by
   // reference, not by value.
   ErrorResult(const ErrorResult&) = delete;
+  void operator=(const ErrorResult&) = delete;
   void ThrowErrorWithMessage(va_list ap, const dom::ErrNum errorNumber,
                              nsresult errorType);
 };