Implement device_orientation::Provider.

chrome/browser/device_orientation/provider.h

 // Copyright (c) 2010 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 CHROME_BROWSER_DEVICE_ORIENTATION_PROVIDER_H_
 #define CHROME_BROWSER_DEVICE_ORIENTATION_PROVIDER_H_
 
 #include "base/logging.h"
 #include "base/ref_counted.h"
 
 namespace device_orientation {
 
 class Orientation;
 
-class Provider : public base::RefCounted<Provider> {
+class Provider : public base::RefCountedThreadSafe<Provider> {
  public:
   class Observer {
    public:
     // Called when the orientation changes.
     // An Observer must not synchronously call Provider::RemoveObserver
     // or Provider::AddObserver when this is called.
     virtual void OnOrientationUpdate(const Orientation& orientation) = 0;
   };
 
   // Returns a pointer to the singleton instance of this class.
   // The caller should store the returned pointer in a scoped_refptr.
   // The Provider instance is lazily constructed when GetInstance() is called,
   // and destructed when the last scoped_refptr referring to it is destructed.
   static Provider* GetInstance();
 
   // Inject a mock Provider for testing. Only a weak pointer to the injected
   // object will be held by Provider, i.e. it does not itself contribute to the
   // injected object's reference count.
   static void SetInstanceForTests(Provider* provider);
 
-  virtual void AddObserver(Observer* observer) {}
-  virtual void RemoveObserver(Observer* observer) {}
+  virtual void AddObserver(Observer* observer) = 0;
+  virtual void RemoveObserver(Observer* observer) = 0;
 
  protected:
   Provider() {}
   virtual ~Provider() {
     DCHECK(instance_ == this);
     instance_ = NULL;
   }
 
  private:
-  friend class base::RefCounted<Provider>;
+  friend class base::RefCountedThreadSafe<Provider>;
   static Provider* instance_;
 
   DISALLOW_COPY_AND_ASSIGN(Provider);
 };
 
 }  // namespace device_orientation
 
 #endif  // CHROME_BROWSER_DEVICE_ORIENTATION_PROVIDER_H_