cros: Apply the Init() model to BrightnessLibrary API.

chrome/browser/chromeos/cros/brightness_library.h

 // Copyright (c) 2011 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_CHROMEOS_CROS_BRIGHTNESS_LIBRARY_H_
 #define CHROME_BROWSER_CHROMEOS_CROS_BRIGHTNESS_LIBRARY_H_
 #pragma once
 
 namespace chromeos {
 
 class BrightnessLibrary {
  public:
   class Observer {
    public:
     virtual void BrightnessChanged(int level, bool user_initiated) = 0;
   };
 
   virtual ~BrightnessLibrary() {}
 
+  virtual void Init() = 0;
+
+  virtual void AddObserver(Observer* observer) = 0;
+  virtual void RemoveObserver(Observer* observer) = 0;
+
   virtual void DecreaseScreenBrightness(bool allow_off) = 0;
   virtual void IncreaseScreenBrightness() = 0;
 
-  virtual void AddObserver(Observer* observer) = 0;
-  virtual void RemoveObserver(Observer* observer) = 0;
-
   // Factory function, creates a new instance and returns ownership.
   // For normal usage, access the singleton via CrosLibrary::Get().
   static BrightnessLibrary* GetImpl(bool stub);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_CROS_BRIGHTNESS_LIBRARY_H_