Prepare for removal of source_filter.gypi:

runtime/bin/test_extension_dllmain_win.cc

Index: runtime/bin/test_extension_dllmain_win.cc
===================================================================
--- runtime/bin/test_extension_dllmain_win.cc	(revision 18775)
+++ runtime/bin/test_extension_dllmain_win.cc	(working copy)
@@ -2,11 +2,16 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+#include "platform/globals.h"
+#if defined(TARGET_OS_WINDOWS)
+
 #define WIN32_LEAN_AND_MEAN
-#include <windows.h>
+#include <windows.h>  // NOLINT
 
 BOOL APIENTRY DllMain(HMODULE module,
                       DWORD  reason,
                       LPVOID reserved) {
   return true;
 }
+
+#endif  // defined(TARGET_OS_WINDOWS)