ifdef out unused thread_pool.h in broker_services.cc to prevent dependencies.

diff --git a/sandbox/win/src/broker_services.cc b/sandbox/win/src/broker_services.cc
--- a/sandbox/win/src/broker_services.cc
+++ b/sandbox/win/src/broker_services.cc
@@ -14,7 +14,9 @@
 #include "base/containers/contains.h"
 #include "base/memory/ptr_util.h"
 #include "base/notreached.h"
+#if !defined(MOZ_SANDBOX)
 #include "base/task/thread_pool.h"
+#endif  // !defined(MOZ_SANDBOX)
 #include "base/threading/platform_thread.h"
 #include "base/win/access_token.h"
 #include "base/win/current_module.h"