[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 574e62925ace61d2cb8d60c36e767eae41599438..1fc5b3532d1007dec84a94267c0b181234f36388 100644
--- a/chrome/browser/extensions/api/system_info/system_info_provider.h
+++ b/chrome/browser/extensions/api/system_info/system_info_provider.h
@@ -29,6 +29,13 @@ namespace extensions {
 //
 // Template parameter T is the system information type. It could be the
 // structure type generated by IDL parser.
+//
+// The class member |info_| is accessed on multiple threads, but that the whole
+// class is being guarded by SystemInfoProvider.
+//
+// |info_| is accessed on the UI thread while |is_waiting_for_completion_| is
+// false and on the BlockingPool under |worker_pool_token_| while
+// |is_waiting_for_completion_| is true.
 template<class T>
 class SystemInfoProvider
     : public base::RefCountedThreadSafe<SystemInfoProvider<T> > {
@@ -68,33 +75,49 @@ class SystemInfoProvider
 
     is_waiting_for_completion_ = true;
 
+    StartQueryInfoImpl();
+  }
+
+ protected:
+  // Default implementation of querying system information.
+  //
+  // While overriding, there are two things need to do:
+  //   1). Bind custom callback function for query system information.
+  //   2). Post the custom task to blocking pool.
+  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.
+  //
+  // The parameter query_callback should invoke QueryInfo directly or indirectly
+  // to query the system information and return to UI thread when the query is
+  // completed.
+  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
   // false is returned.
+  // TODO(Haojian): Remove the parameter T-typed pointer, replacing with void
+  // QueryInfo().
   virtual bool QueryInfo(T* info) = 0;
 
-  virtual void QueryOnWorkerPool() {
-    bool success = QueryInfo(&info_);
-    content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
-        base::Bind(&SystemInfoProvider<T>::OnQueryCompleted, this, success));
-  }
-
   // Called on UI thread. The |success| parameter means whether it succeeds
   // to get the information.
-  virtual void OnQueryCompleted(bool success) {
+  void OnQueryCompleted(bool success) {
     DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 
     while (!callbacks_.empty()) {
@@ -104,7 +127,7 @@ class SystemInfoProvider
     }
 
     is_waiting_for_completion_ = false;
- }
+  }
 
   // Template function for creating the single shared provider instance.
   // Template paramter I is the type of SystemInfoProvider implementation.
@@ -121,6 +144,14 @@ class SystemInfoProvider
   T info_;
 
  private:
+  // TODO(Haojian): Use PostBlockingPoolTaskAndReply to avoid unnecessary
+  // trampolines trip.
+  void QueryOnWorkerPool() {
+    bool success = QueryInfo(&info_);
+    content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
+        base::Bind(&SystemInfoProvider<T>::OnQueryCompleted, this, success));
+  }
+
   // The single shared provider instance. We create it only when needed.
   static typename base::LazyInstance<
       scoped_refptr<SystemInfoProvider<T> > > single_shared_provider_;