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 ece683b339239a46d62b9baee10638a9cfe66b43..8774603d81f8bbf76a2acefbf2ebdb5f2240b277 100644
--- a/ios/web_view/internal/cwv_scroll_view.mm
+++ b/ios/web_view/internal/cwv_scroll_view.mm
@@ -66,6 +66,14 @@
   _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: