Have PlatformEventObserver implementations orderly stop on destruction.

content/public/renderer/platform_event_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.
 
 #ifndef CONTENT_PUBLIC_RENDERER_PLATFORM_EVENT_OBSERVER_H_
 #define CONTENT_PUBLIC_RENDERER_PLATFORM_EVENT_OBSERVER_H_
 
 #include "base/logging.h"
 #include "content/public/renderer/render_process_observer.h"
 #include "content/public/renderer/render_thread.h"
@@ skipping 42 matching lines @@
   // Creates a PlatformEventObserver that registers to the RenderThread in order
   // to intercept the received IPC messages (via OnControlMessageReceived). If
   // |thread| is null, it will not register.
   explicit PlatformEventObserver(RenderThread* thread)
       : is_observing_(false),
         listener_(0) {
     if (thread)
       thread->AddObserver(this);
   }
 
-  // The observer will automatically stop observing when destroyed in case it
-  // did not stop before.
+  // The observer must automatically stop observing when destroyed in case it
+  // did not stop before. Implementations of PlatformEventObserver must do
+  // so by calling StopIfObserving() from their destructors.
   virtual ~PlatformEventObserver() {
-    if (is_observing())
-      Stop();
+    // If this assert fails, the derived destructor failed to invoke
+    // StopIfObserving().
+    DCHECK(!is_observing());
   }
 
   // Called when a new IPC message is received. Must be used to listen to the
   // responses from the browser process if any expected.
   virtual bool OnControlMessageReceived(const IPC::Message& msg) OVERRIDE {
     return false;
   }
 
   // Start observing. Will request the browser process to start listening to the
   // events. |listener| will receive any response from the browser process.
@@ skipping 20 matching lines @@
   // This method is expected to send an IPC to the browser process to let it
   // know that it should start observing.
   // It is expected for subclasses to override it.
   virtual void SendStartMessage() = 0;
 
   // This method is expected to send an IPC to the browser process to let it
   // know that it should start observing.
   // It is expected for subclasses to override it.
   virtual void SendStopMessage() = 0;
 
+  // Implementations of PlatformEventObserver must call StopIfObserving()
+  // from their destructor to shutdown in an orderly manner.
+  // (As Stop() calls a virtual method, it cannot be handled by
+  // ~PlatformEventObserver.)
+  void StopIfObserving() {
+    if (is_observing())
+      Stop();
+  }
+
   bool is_observing() const {
     return is_observing_;
   }
 
   ListenerType* listener() {
     return listener_;
   }
 
  private:
   bool is_observing_;
   ListenerType* listener_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformEventObserver);
 };
 
 } // namespace content
 
 #endif // CONTENT_PUBLIC_RENDERER_PLATFORM_EVENT_OBSERVER_H_