Bug 1417655: Mark private operator= delete for NullPrincipalURI since there is a private copy constr.r=bholley

Differential Revision: https://phabricator.services.mozilla.com/D65371

--HG--
extra : moz-landing-system : lando

caps/NullPrincipalURI.h

@@ -45,6 +45,7 @@ class NullPrincipalURI final : public nsIURI, public nsISizeOf {
 
  private:
   NullPrincipalURI(const NullPrincipalURI& aOther);
+  void operator=(const NullPrincipalURI& aOther) = delete;
 
   ~NullPrincipalURI() {}