Update ui::PointerEvent to support mouse wheel and capture change events.

ui/views/pointer_watcher.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_POINTER_WATCHER_H_
 #define UI_VIEWS_POINTER_WATCHER_H_
 
 #include "base/macros.h"
 #include "ui/views/views_export.h"
 
 namespace gfx {
 class Point;
 }
 
 namespace ui {
 class PointerEvent;
 }
 
 namespace views {
 class Widget;
 
 // An interface for read-only observation of pointer events (in particular, the
 // events cannot be marked as handled). Only certain event types are supported.
 // The |target| is the top-level widget that will receive the event, if any.
 // To reduce IPC traffic from the window server, move events are not provided
 // unless the app specifically requests them.
 // NOTE: On mus this allows observation of events outside of windows owned
 // by the current process, in which case the |target| will be null. On mus
 // event.target() is always null.
+// NOTE: Mouse capture change events are sent through OnPointerEventObserved and
+// its |target| is always null.
 class VIEWS_EXPORT PointerWatcher {
  public:
   PointerWatcher() {}
 
   virtual void OnPointerEventObserved(const ui::PointerEvent& event,
                                       const gfx::Point& location_in_screen,
                                       Widget* target) = 0;
 
-  // PointerWatcher is informed of capture changes as it's common to create a
-  // MouseEvent of type ui::ET_MOUSE_CAPTURE_CHANGED on capture change.
-  virtual void OnMouseCaptureChanged() {}
-
  protected:
   virtual ~PointerWatcher() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PointerWatcher);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_POINTER_WATCHER_H_