Fixed crash issue on Flutter >= 0.10.2

Author: dinhha

ios/Classes/FlutterWebviewPlugin.m

@@ -15,7 +15,7 @@ + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
                methodChannelWithName:CHANNEL_NAME
                binaryMessenger:[registrar messenger]];
 
-    UIViewController *viewController = (UIViewController *)registrar.messenger;
+    UIViewController *viewController = [UIApplication sharedApplication].delegate.window.rootViewController;
     FlutterWebviewPlugin* instance = [[FlutterWebviewPlugin alloc] initWithViewController:viewController];
 
     [registrar addMethodCallDelegate:instance channel:channel];