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

mojo/public/c/system/handle.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 Mojo system handle-related declarations/definitions.
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_HANDLE_H_
 #define MOJO_PUBLIC_C_SYSTEM_HANDLE_H_
@@ skipping 37 matching lines @@
 // Note: This struct is not extensible (and only has 32-bit quantities), so it's
 // 32-bit-aligned.
 MOJO_STATIC_ASSERT(MOJO_ALIGNOF(uint32_t) == 4, "uint32_t has weird alignment");
 struct MOJO_ALIGNAS(4) MojoHandleSignalsState {
   MojoHandleSignals satisfied_signals;
   MojoHandleSignals satisfiable_signals;
 };
 MOJO_STATIC_ASSERT(sizeof(MojoHandleSignalsState) == 8,
                    "MojoHandleSignalsState has wrong size");
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 // Closes the given |handle|.
 //
 // Returns:
 //   |MOJO_RESULT_OK| on success.
 //   |MOJO_RESULT_INVALID_ARGUMENT| if |handle| is not a valid handle.
 //
 // Concurrent operations on |handle| may succeed (or fail as usual) if they
 // happen before the close, be cancelled with result |MOJO_RESULT_CANCELLED| if
 // they properly overlap (this is likely the case with |MojoWait()|, etc.), or
 // fail with |MOJO_RESULT_INVALID_ARGUMENT| if they happen after.
 MojoResult MojoClose(MojoHandle handle);
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_HANDLE_H_