Have PlatformEventObserver implementations orderly stop on destruction.

content/renderer/screen_orientation/screen_orientation_observer.h

 // Copyright 2014 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.
 
 #include "content/public/renderer/platform_event_observer.h"
 
 namespace content {
 
 // ScreenOrientationObserver is a helper class implementing the
 // PlatformEventObserver template with blink::WebPlatformEventListener, which is
 // a pure virtual class because it doesn't expect listeners.
 // It is implemented on top of PlatformEventObserver to make sure it follows a
 // common code path even though it only uses part of this code path: it is not
 // expected to receive messages back but should send messages to start/stop
 // listening.
 // The intent of this class is to make sure that platforms that can't listen for
 // display rotations at a marginal cost can be told when to actually do it.
 class ScreenOrientationObserver
     : public PlatformEventObserver<blink::WebPlatformEventListener> {
  public:
   ScreenOrientationObserver();
+  virtual ~ScreenOrientationObserver();
 
   // Overriding this method just to make sure |listener| is always null.
   virtual void Start(blink::WebPlatformEventListener* listener) OVERRIDE;
 
  protected:
   virtual void SendStartMessage() OVERRIDE;
   virtual void SendStopMessage() OVERRIDE;
 };
 
 }; // namespace content