Prepare for removal of source_filter.gypi:

runtime/bin/extensions_win.cc

Index: runtime/bin/extensions_win.cc
===================================================================
--- runtime/bin/extensions_win.cc	(revision 18775)
+++ runtime/bin/extensions_win.cc	(working copy)
@@ -2,6 +2,9 @@
 // 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)
+
 #include "bin/extensions.h"
 #include "bin/utils.h"
 
@@ -19,3 +22,5 @@
 void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
   return GetProcAddress(reinterpret_cast<HMODULE>(lib_handle), symbol);
 }
+
+#endif  // defined(TARGET_OS_WINDOWS)