[SystemInfo API] Rewrite storage info provider using storage monitor impl

chrome/browser/extensions/api/system_info/system_info_provider.h

Index: chrome/browser/extensions/api/system_info/system_info_provider.h
diff --git a/chrome/browser/extensions/api/system_info/system_info_provider.h b/chrome/browser/extensions/api/system_info/system_info_provider.h
index 849075dfa193181ec06f169d8f1438ec7440fe55..66f03aa4909fa67584f6ab67f7cdb88734b592dd 100644
--- a/chrome/browser/extensions/api/system_info/system_info_provider.h
+++ b/chrome/browser/extensions/api/system_info/system_info_provider.h
@@ -40,10 +40,9 @@ class SystemInfoProvider
   typedef base::Callback<void(const T&, bool)> QueryInfoCompletionCallback;
   typedef std::queue<QueryInfoCompletionCallback> CallbackQueue;
 
-  SystemInfoProvider()
-    : is_waiting_for_completion_(false) {
+  SystemInfoProvider() : is_waiting_for_completion_(false) {
     worker_pool_token_ =
-      content::BrowserThread::GetBlockingPool()->GetSequenceToken();
+        content::BrowserThread::GetBlockingPool()->GetSequenceToken();
   }
 
   virtual ~SystemInfoProvider() {}
@@ -62,24 +61,34 @@ class SystemInfoProvider
     DCHECK(!callback.is_null());
 
     callbacks_.push(callback);
-
     if (is_waiting_for_completion_)
       return;
-
     is_waiting_for_completion_ = true;
 
+    StartQueryInfoImpl();
+  }
+
+ protected:
+  // Default implementation of querying system information.
+  virtual void StartQueryInfoImpl() {
+    base::Closure callback =
+        base::Bind(&SystemInfoProvider<T>::QueryOnWorkerPool, this);
+    PostQueryTaskToBlockingPool(FROM_HERE, callback);
+  }
+
+  // Post a task to blocking pool for information querying.
+  void PostQueryTaskToBlockingPool(const tracked_objects::Location& from_here,
+                                   const base::Closure& query_callback) {
+    DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
     base::SequencedWorkerPool* worker_pool =
         content::BrowserThread::GetBlockingPool();
     // The query task posted to the worker pool won't block shutdown, and any
     // running query task at shutdown time will be ignored.
     worker_pool->PostSequencedWorkerTaskWithShutdownBehavior(
-          worker_pool_token_,
-          FROM_HERE,
-          base::Bind(&SystemInfoProvider<T>::QueryOnWorkerPool, this),
+          worker_pool_token_, from_here, query_callback,
           base::SequencedWorkerPool::CONTINUE_ON_SHUTDOWN);
   }
 
- protected:
   // Query the system information synchronously and output the result to the
   // |info| parameter. The |info| contents MUST be reset firstly in its
   // platform specific implementation. Return true if it succeeds, otherwise