Fix a crash bug on deallocation of CWVWebView caused by CWVScrollView.

ios/web_view/internal/cwv_scroll_view.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.
 
 #import "ios/web_view/public/cwv_scroll_view.h"
 
 #import "ios/web/public/web_state/ui/crw_web_view_scroll_view_proxy.h"
 #import "ios/web_view/internal/cwv_scroll_view_internal.h"
 #import "ios/web_view/public/cwv_scroll_view_delegate.h"
 
@@ skipping 48 matching lines @@
 }
 
 - (UIEdgeInsets)contentInset {
   return _proxy.contentInset;
 }
 
 - (void)setContentInset:(UIEdgeInsets)contentInset {
   _proxy.contentInset = contentInset;
 }
 
+#pragma mark - NSObject
+
+- (void)dealloc {
+  // Explicitly set |proxy| to nil to remove |self| from |proxy|'s observers.
+  // Otherwise |proxy| will keep a dangling pointer to |self| and cause SEGV.
+  self.proxy = nil;

[Eugene But (OOO till 7-30), 2017/04/28 07:06:23]

_proxy = nil;

https://google.github.io/styleguide/objcguide.xml#Avoid_Accessors_During_init...

[Hiroshi Ichikawa, 2017/04/28 08:47:41]

The goal here is to call [_proxy removeObserver:self], so _proxy = nil doesn't
achieve the goal. I updated the code to call -removeObserver: directly.

[Eugene But (OOO till 7-30), 2017/04/28 10:08:57]

Thanks!

+}
+
 #pragma mark - CRWWebViewScrollViewObserver
 
 - (void)webViewScrollViewWillBeginDragging:
     (CRWWebViewScrollViewProxy*)webViewScrollViewProxy {
   SEL selector = @selector(webViewScrollViewWillBeginDragging:);
   if ([_delegate respondsToSelector:selector]) {
     [_delegate scrollViewWillBeginDragging:self];
   }
 }
 - (void)webViewScrollViewWillEndDragging:
@@ skipping 24 matching lines @@
     [_delegate scrollViewDidEndDecelerating:self];
   }
 }
 
 - (void)webViewScrollViewDidResetContentSize:
     (CRWWebViewScrollViewProxy*)webViewScrollViewProxy {
   self.contentSize = _proxy.contentSize;
 }
 
 @end