Weak engine is strong

Previously we held a scoped_nsobject<FlutterEngine> as a strong
reference ivar. This restores that behaviour.

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -156,6 +156,7 @@ @implementation FlutterViewController {
   // TODO(cbracken): https://github.com/flutter/flutter/issues/137801
   // Eliminate once we can use weak pointers in platform_view_ios.h.
   std::unique_ptr<fml::WeakNSObjectFactory<FlutterViewController>> _weakFactory;
+  FlutterEngine* _engine;
 
   flutter::ViewportMetrics _viewportMetrics;
   MouseState _mouseState;
@@ -307,6 +308,10 @@ - (void)performCommonViewControllerInitialization {
   [self setUpNotificationCenterObservers];
 }
 
+- (FlutterEngine*)engine {
+  return _engine;
+}
+
 - (fml::WeakNSObject<FlutterViewController>)getWeakNSObject {
   return _weakFactory->GetWeakNSObject();
 }