Add MOJO_{BEGIN,END}_EXTERN_C macros, and use them.

mojo/public/c/system/main.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_MAIN_H_
 #define MOJO_PUBLIC_C_SYSTEM_MAIN_H_
 
 #include "mojo/public/c/system/handle.h"
+#include "mojo/public/c/system/macros.h"
 #include "mojo/public/c/system/result.h"
 
 // Implement MojoMain directly as the entry point for an application.
 //
 // MojoResult MojoMain(MojoHandle application_request) {
 //   ...
 // }
 
-#if defined(__cplusplus)
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 __attribute__((visibility("default"))) MojoResult MojoMain(
     MojoHandle application_request);
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_MAIN_H_