Adds code to isolate use-after-free in Views

ui/views/view_observer.h

Index: ui/views/view_observer.h
diff --git a/ui/views/view_observer.h b/ui/views/view_observer.h
index c1d69f92cc313e4d73fdb4b6b1124a54ef588e50..5876588d0e5767f9876fa41edd9ccbcb991577d8 100644
--- a/ui/views/view_observer.h
+++ b/ui/views/view_observer.h
@@ -33,6 +33,9 @@ class VIEWS_EXPORT ViewObserver {
   // parent view. |view| is the child view being moved.
   virtual void OnChildViewReordered(View* view) {}
 
+  // Called from ~View.
+  virtual void OnViewIsDeleting(View* observed_view) {}

[msw, 2017/03/14 20:37:55]

nit: Make the override decl/def match.

[sky, 2017/03/14 21:00:59]

I don't think the style guide requires that, and in the context of FocusManager
using |observed_view| feels overly verbose.

[msw, 2017/03/14 21:15:20]

fair enough.
I'd say the same for this ViewObserver interface signature.

Still, it's just a nit, the CL is fine as-is.

+
  protected:
   virtual ~ViewObserver() {}
 };