Adds code to isolate use-after-free in Views

ui/views/view_observer.h

 // Copyright 2016 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.
 
 #ifndef UI_VIEWS_VIEW_OBSERVER_H_
 #define UI_VIEWS_VIEW_OBSERVER_H_
 
 #include "ui/views/views_export.h"
 
 namespace views {
@@ skipping 15 matching lines @@
   virtual void OnViewVisibilityChanged(View* view) {}
 
   virtual void OnViewEnabledChanged(View* view) {}
 
   virtual void OnViewBoundsChanged(View* view) {}
 
   // Invoked whenever a child is moved to another index. This is called on the
   // 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() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_VIEW_OBSERVER_H_