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

ios/web_view/internal/cwv_scroll_view.mm

Index: ios/web_view/internal/cwv_scroll_view.mm
diff --git a/ios/web_view/internal/cwv_scroll_view.mm b/ios/web_view/internal/cwv_scroll_view.mm
index ceec039a0322fbd92eb14e51b3afb10ae983aeaa..53dc5d8cb22857e8edd9ced179bc94dcc5e9da57 100644
--- a/ios/web_view/internal/cwv_scroll_view.mm
+++ b/ios/web_view/internal/cwv_scroll_view.mm
@@ -62,6 +62,14 @@
   [_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: