Add CRWNativeContent willBeDismissed.

ios/web/web_state/ui/crw_web_controller_container_view.mm

 // Copyright 2015 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.
 
 #import "ios/web/web_state/ui/crw_web_controller_container_view.h"
 
 #include "base/ios/weak_nsobject.h"
 #include "base/logging.h"
 #include "base/mac/scoped_nsobject.h"
 #import "ios/web/public/web_state/ui/crw_content_view.h"
@@ skipping 154 matching lines @@
   }
 }
 
 - (id<CRWNativeContent>)nativeController {
   return _nativeController.get();
 }
 
 - (void)setNativeController:(id<CRWNativeContent>)nativeController {
   if (![_nativeController isEqual:nativeController]) {
     base::WeakNSProtocol<id> oldController(_nativeController);
+    if ([oldController respondsToSelector:@selector(willBeDismissed)]) {
+      [oldController willBeDismissed];
+    }
     [[oldController view] removeFromSuperview];
     _nativeController.reset([nativeController retain]);
     // TODO(crbug.com/503297): Re-enable this DCHECK once native controller
     // leaks are fixed.
     //    DCHECK(!oldController);
   }
 }
 
 - (CRWContentView*)transientContentView {
   return _transientContentView.get();
@@ skipping 150 matching lines @@
   else
     self.toolbarContainerView = nil;
 }
 
 - (void)removeAllToolbars {
   // Resetting the property will remove the toolbars from the hierarchy.
   self.toolbarContainerView = nil;
 }
 
 @end