Add an IO thread index bitmap to disk cache.

net/disk_cache/in_flight_io.h

Index: net/disk_cache/in_flight_io.h
diff --git a/net/disk_cache/in_flight_io.h b/net/disk_cache/in_flight_io.h
index 2ba7517cec68043f971197a5eb0dc69733afd42a..f61117938c70da824533bc6ba3ed596fe733867d 100644
--- a/net/disk_cache/in_flight_io.h
+++ b/net/disk_cache/in_flight_io.h
@@ -52,6 +52,8 @@ class BackgroundIO : public base::RefCountedThreadSafe<BackgroundIO> {
   // thread.
   void NotifyController();
 
+  scoped_refptr<base::MessageLoopProxy> GetCallbackThread();
+
   int result_;  // Final operation result.
 
  private:
@@ -108,6 +110,10 @@ class InFlightIO {
   // the one performing the call.
   void InvokeCallback(BackgroundIO* operation, bool cancel_task);
 
+  scoped_refptr<base::MessageLoopProxy> callback_thread() {
+    return callback_thread_;
+  }
+
  protected:
   // This method is called to signal the completion of the |operation|. |cancel|
   // is true if the operation is being cancelled. This method is called on the