[CRD iOS] Fix the leaky RemotingClient

remoting/ios/app/host_view_controller.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 #import "remoting/ios/app/host_view_controller.h"
 
@@ skipping 93 matching lines @@
       addObserver:self
          selector:@selector(keyboardWillHide:)
              name:UIKeyboardWillHideNotification
            object:nil];
 }
 
 - (void)viewWillDisappear:(BOOL)animated {
   [super viewWillDisappear:animated];
 
   _clientGestures = nil;
+  _client = nil;
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
 - (void)viewDidLayoutSubviews {
   [super viewDidLayoutSubviews];
 
   if (((GLKView*)self.view).context != nil) {
     // If the context is not set yet, the view size will be set in
     // viewDidAppear.
     [_client surfaceChanged:self.view.bounds];
@@ skipping 91 matching lines @@
     [self dismissViewControllerAnimated:YES completion:nil];
   };
   [alert addAction:[UIAlertAction actionWithTitle:@"Disconnect"
                                             style:UIAlertActionStyleCancel
                                           handler:disconnectHandler]];
 
   [self presentViewController:alert animated:YES completion:nil];
 }
 
 @end