Map iOS touches to Flutter view coordinate system (flutter#4425)

This change ensures that touches are mapped to the Flutter view
co-ordinate system. In the case of a Flutter view that doesn't share the
same origin and orientation as the screen co-ordinate system, touches
were appled in the wrong location. This bug affected Flutter views whose
origin was not the screen origin and Flutter apps running with the
in-call status bar on iPhones other than the iPhone X.

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

@@ -483,7 +483,7 @@ - (void)dispatchTouches:(NSSet*)touches phase:(UITouchPhase)phase {
     }
 
     FXL_DCHECK(device_id != 0);
-    CGPoint windowCoordinates = [touch locationInView:nil];
+    CGPoint windowCoordinates = [touch locationInView:self.view];
 
     blink::PointerData pointer_data;
     pointer_data.Clear();