Reland r113263 rebased after revert of r113249.

net/disk_cache/backend_impl.cc

Index: net/disk_cache/backend_impl.cc
diff --git a/net/disk_cache/backend_impl.cc b/net/disk_cache/backend_impl.cc
index 4ffac86384684d1eb497620e48994093b860b9c7..1443f7faa7a8cf54355e06c90b36420942c449ca 100644
--- a/net/disk_cache/backend_impl.cc
+++ b/net/disk_cache/backend_impl.cc
@@ -1437,6 +1437,13 @@ int BackendImpl::OpenNextEntry(void** iter, Entry** next_entry,
   return net::ERR_IO_PENDING;
 }
 
+int BackendImpl::OpenNextEntry(void** iter, Entry** next_entry,
+                               const net::CompletionCallback& callback) {
+  DCHECK(!callback.is_null());
+  background_queue_.OpenNextEntry(iter, next_entry, callback);
+  return net::ERR_IO_PENDING;
+}
+
 void BackendImpl::EndEnumeration(void** iter) {
   background_queue_.EndEnumeration(*iter);
   *iter = NULL;