Polish the SystemInfoProvider template code and refactor StorageInfoProvider based on it

chrome/browser/extensions/system_info_provider.h

 // Copyright (c) 2012 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_EXTENSIONS_SYSTEM_INFO_PROVIDER_H_
 #define CHROME_BROWSER_EXTENSIONS_SYSTEM_INFO_PROVIDER_H_
 
 #include <queue>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 29 matching lines @@
   typedef std::queue<QueryInfoCompletionCallback> CallbackQueue;
 
   SystemInfoProvider()
     : is_waiting_for_completion_(false) {
     worker_pool_token_ =
       content::BrowserThread::GetBlockingPool()->GetSequenceToken();
   }
 
   virtual ~SystemInfoProvider() {}
 
-  // Static method to get the single shared instance. Should be implemented
-  // in the impl file for each kind of provider.
-  static SystemInfoProvider<T>* Get();
-
   // For testing
   static void InitializeForTesting(SystemInfoProvider<T>* provider) {
     DCHECK(provider != NULL);
     single_shared_provider_.Get().reset(provider);
   }
 
   // Start to query the system information. Should be called on UI thread.
   // The |callback| will get called once the query is completed.
   void StartQueryInfo(const QueryInfoCompletionCallback& callback) {
     DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
@@ skipping 42 matching lines @@
       callback.Run(info_, success);
       callbacks_.pop();
     }
 
     is_waiting_for_completion_ = false;
  }
 
   // Template function for creating the single shared provider instance.
   // Template paramter I is the type of SystemInfoProvider implementation.
   template<class I>
-  static SystemInfoProvider<T>* GetInstance() {
+  static I* GetInstance() {
     if (!single_shared_provider_.Get().get()) {
       I* impl = new I();
       single_shared_provider_.Get().reset(impl);
     }
-    return single_shared_provider_.Get().get();
+    return static_cast<I*>(single_shared_provider_.Get().get());
   }
 
   // The latest information filled up by QueryInfo implementation. Here we
   // assume the T is disallowed to copy constructor, aligns with the structure
   // type generated by IDL parser.
   T info_;
 
  private:
   // The single shared provider instance. We create it only when needed.
   static typename base::LazyInstance<
@@ skipping 14 matching lines @@
 };
 
 // Static member intialization.
 template<class T>
 typename base::LazyInstance<scoped_ptr<SystemInfoProvider<T> > >
   SystemInfoProvider<T>::single_shared_provider_ = LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SYSTEM_INFO_PROVIDER_H_