Have PlatformEventObserver implementations orderly stop on destruction.

content/renderer/screen_orientation/screen_orientation_observer.cc

 // 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/renderer/screen_orientation/screen_orientation_observer.h"
 
 #include "content/common/screen_orientation_messages.h"
 
 namespace content {
 
 ScreenOrientationObserver::ScreenOrientationObserver() {
 }
 
+ScreenOrientationObserver::~ScreenOrientationObserver() {
+  StopIfObserving();
+}
+
 void ScreenOrientationObserver::Start(
     blink::WebPlatformEventListener* listener) {
   // This should never be called with a proper listener.
   CHECK(listener == 0);
   PlatformEventObserver<blink::WebPlatformEventListener>::Start(0);
 }
 
 void ScreenOrientationObserver::SendStartMessage() {
   RenderThread::Get()->Send(new ScreenOrientationHostMsg_StartListening());
 }
 
 void ScreenOrientationObserver::SendStopMessage() {
   RenderThread::Get()->Send(new ScreenOrientationHostMsg_StopListening());
 }
 
 } // namespace content