Forward declaration of class instead of including header in net/base as per chromium's guidelines

https://www.chromium.org/developers/coding-style/cpp-dos-and-donts#TOC-Forward-declare-classes-instead-of-including-headers.

Review URL: https://codereview.chromium.org/1135793006

Cr-Commit-Position: refs/heads/master@{#330708}

net/base/directory_lister.cc

@@ -10,8 +10,9 @@
 #include "base/files/file_enumerator.h"
 #include "base/files/file_util.h"
 #include "base/i18n/file_util_icu.h"
+#include "base/location.h"
 #include "base/logging.h"
-#include "base/message_loop/message_loop.h"
+#include "base/message_loop/message_loop_proxy.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/worker_pool.h"
 #include "net/base/net_errors.h"

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

net/base/directory_lister_unittest.cc

@@ -11,7 +11,6 @@
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/i18n/file_util_icu.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "net/base/directory_lister.h"