Polish the SystemInfoProvider template code and refactor StorageInfoProvider based on it

chrome/browser/extensions/system_info_provider.h

Index: chrome/browser/extensions/system_info_provider.h
diff --git a/chrome/browser/extensions/system_info_provider.h b/chrome/browser/extensions/system_info_provider.h
index 38fac9e5108be049e3c1932bb9035b120f9fce9f..d9057d5ceda4713d28dd488284d6f7dbf7ecc6c2 100644
--- a/chrome/browser/extensions/system_info_provider.h
+++ b/chrome/browser/extensions/system_info_provider.h
@@ -47,10 +47,6 @@ class SystemInfoProvider {
 
   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();
-

[Hongbo Min, 2012/08/23 15:05:05]

The Get() is now moved down to the SystemInfoProvider<T> derived class for that
the specific system info provider may have their own additional interfaces
besides QueryInfo. For example, storage info provider also needs an interface to
query the storage unit information with the storage identifier. 

So we need to make sure Get() returns the specific implementation of
SystemInfoProvider<T> class. Otherwise, the Get() has to do downcast operation
from SystemInfoProvider<T> to XXXInfoProvider (e.g. StorageInfoProvider).

   // For testing
   static void InitializeForTesting(SystemInfoProvider<T>* provider) {
     DCHECK(provider != NULL);
@@ -113,12 +109,12 @@ class SystemInfoProvider {
   // 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