[net/base] Forward declaration of classes instead of including header

net/base/directory_lister.h

Index: net/base/directory_lister.h
diff --git a/net/base/directory_lister.h b/net/base/directory_lister.h
index 02f24dfbe3448c8bac5cef5ab79a5a6f6daf48df..2b43c1d51ac0ac902bb2b5b87189c036bad5819a 100644
--- a/net/base/directory_lister.h
+++ b/net/base/directory_lister.h
@@ -13,9 +13,12 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/message_loop/message_loop_proxy.h"
 #include "net/base/net_export.h"
 
+namespace base {
+class MessageLoopProxy;
+}
+
 namespace net {
 
 // This class provides an API for asynchronously listing the contents of a