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 44 matching lines @@
 }
 
 - (void)setContentInset:(UIEdgeInsets)contentInset {
   _proxy.contentInset = contentInset;
 }
 
 - (void)addGestureRecognizer:(UIGestureRecognizer*)gestureRecognizer {
   [_proxy addGestureRecognizer:gestureRecognizer];
 }
 
+#pragma mark - NSObject
+
+- (void)dealloc {
+  // Removes |self| from |_proxy|'s observers. Otherwise |_proxy| will keep a
+  // dangling pointer to |self| and cause SEGV later.
+  [_proxy removeObserver:self];
+}
+
 #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