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

mojo/public/c/system/wait.h

 // Copyright 2016 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.
 
 // This file contains basic functions common to different Mojo system APIs.
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_WAIT_H_
 #define MOJO_PUBLIC_C_SYSTEM_WAIT_H_
 
 #include "mojo/public/c/system/handle.h"
+#include "mojo/public/c/system/macros.h"
 #include "mojo/public/c/system/result.h"
 #include "mojo/public/c/system/time.h"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 // Waits on the given handle until one of the following happens:
 //   - A signal indicated by |signals| is satisfied.
 //   - It becomes known that no signal indicated by |signals| will ever be
 //     satisfied. (See the description of the |MOJO_RESULT_CANCELLED| and
 //     |MOJO_RESULT_FAILED_PRECONDITION| return values below.)
 //   - Until |deadline| has passed.
 //
 // If |deadline| is |MOJO_DEADLINE_INDEFINITE|, this will wait "forever" (until
 // one of the other wait termination conditions is satisfied). If |deadline| is
@@ skipping 65 matching lines @@
 //   |MOJO_RESULT_FAILED_PRECONDITION| if it is or becomes impossible that SOME
 //       |handle[i]| will ever satisfy any of the signals in |signals[i]|.
 MojoResult MojoWaitMany(
     const MojoHandle* handles,
     const MojoHandleSignals* signals,
     uint32_t num_handles,
     MojoDeadline deadline,
     uint32_t* result_index,                          // Optional out
     struct MojoHandleSignalsState* signals_states);  // Optional out
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_WAIT_H_