Add an IO thread index bitmap to disk cache.

net/disk_cache/in_flight_io.cc

Index: net/disk_cache/in_flight_io.cc
diff --git a/net/disk_cache/in_flight_io.cc b/net/disk_cache/in_flight_io.cc
index 51ad986a91632c17836a9fe46035fa661593506f..3c3c1161a1fd4a0f07611ef221ef6750fcb4da35 100644
--- a/net/disk_cache/in_flight_io.cc
+++ b/net/disk_cache/in_flight_io.cc
@@ -48,6 +48,13 @@ void BackgroundIO::NotifyController() {
     controller_->OnIOComplete(this);
 }
 
+scoped_refptr<base::MessageLoopProxy> BackgroundIO::GetCallbackThread() {
+  base::AutoLock lock(controller_lock_);
+  if (!controller_)
+    return NULL;
+  return controller_->callback_thread();
+}
+
 void InFlightIO::WaitForPendingIO() {
   while (!io_list_.empty()) {
     // Block the current thread until all pending IO completes.