Bug 1605130. Make it harder to misuse OwningNonNull::forget. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D57805

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

xpcom/base/OwningNonNull.h

@@ -68,21 +68,6 @@ class MOZ_IS_SMARTPTR_TO_REFCOUNTED OwningNonNull {
   // Don't allow assigning nullptr, it makes no sense
   void operator=(decltype(nullptr)) = delete;
 
-  already_AddRefed<T> forget() {
-#ifdef DEBUG
-    mInited = false;
-#endif
-    return mPtr.forget();
-  }
-
-  template <class U>
-  void forget(U** aOther) {
-#ifdef DEBUG
-    mInited = false;
-#endif
-    mPtr.forget(aOther);
-  }
-
   T& ref() const {
     MOZ_ASSERT(mInited);
     MOZ_ASSERT(mPtr, "OwningNonNull<T> was set to null");
@@ -112,6 +97,17 @@ class MOZ_IS_SMARTPTR_TO_REFCOUNTED OwningNonNull {
     return mPtr;
   }
 
+ private:
+  already_AddRefed<T> forget() {
+#ifdef DEBUG
+    mInited = false;
+#endif
+    return mPtr.forget();
+  }
+
+  // Allow ImplCycleCollectionUnlink to call forget().
+  friend void ImplCycleCollectionUnlink(OwningNonNull<T>& aField);
+
  protected:
   template <typename U>
   void init(U&& aValue) {