[ios] WebStateList owns all WebState it manages.

ios/chrome/browser/ui/overscroll_actions/overscroll_actions_controller.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/chrome/browser/ui/overscroll_actions/overscroll_actions_controller.h"
 
 #import <QuartzCore/QuartzCore.h>
 
 #include <algorithm>
 #include "base/logging.h"
@@ skipping 501 matching lines @@
   [_webViewScrollViewProxy removeObserver:self];
   _webViewScrollViewProxy.reset([[webViewProxy scrollViewProxy] retain]);
   [_webViewScrollViewProxy addObserver:self];
   [self enableOverscrollActions];
 }
 
 - (void)webControllerWillClose:(CRWWebController*)webController {
   [self disableOverscrollActions];
   [_webViewScrollViewProxy removeObserver:self];
   _webViewScrollViewProxy.reset();
-  [webController removeObserver:self];
 }
 
 #pragma mark - Private
 
 - (void)recordMetricForTriggeredAction:(OverscrollAction)action {
   switch (action) {
     case OverscrollAction::NONE:
       UMA_HISTOGRAM_ENUMERATION(kOverscrollActionsHistogram,
                                 OVERSCROLL_ACTION_CANCELED,
                                 OVERSCROLL_ACTION_COUNT);
@@ skipping 369 matching lines @@
       overscrollActionsController:self
                  didTriggerAction:self.overscrollActionView.selectedAction];
 }
 
 - (void)overscrollActionsView:(OverscrollActionsView*)view
       selectedActionDidChange:(OverscrollAction)newAction {
   TriggerHapticFeedbackForSelectionChange();
 }
 
 @end