Prepare for removal of source_filter.gypi:

samples/sample_extension/sample_extension_dllmain_win.cc

Index: samples/sample_extension/sample_extension_dllmain_win.cc
===================================================================
--- samples/sample_extension/sample_extension_dllmain_win.cc	(revision 18775)
+++ samples/sample_extension/sample_extension_dllmain_win.cc	(working copy)
@@ -2,6 +2,8 @@
 // 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.
 
+#if defined(_WIN32)
+
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 
@@ -10,3 +12,5 @@
                       LPVOID reserved) {
   return true;
 }
+
+#endif  // defined(_WIN32)