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

mojo/public/c/system/buffer.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.
 
 // This file contains types/constants and functions specific to buffers (and in
 // particular shared buffers).
 // TODO(vtl): Reorganize this file (etc.) to separate general buffer functions
 // from (shared) buffer creation.
 //
 // Note: This header should be compilable as C.
@@ skipping 74 matching lines @@
 MOJO_STATIC_ASSERT(sizeof(MojoBufferInformation) == 16,
                    "MojoBufferInformation has wrong size");
 
 // |MojoMapBufferFlags|: Used to specify different modes to |MojoMapBuffer()|.
 //   |MOJO_MAP_BUFFER_FLAG_NONE| - No flags; default mode.
 
 typedef uint32_t MojoMapBufferFlags;
 
 #define MOJO_MAP_BUFFER_FLAG_NONE ((MojoMapBufferFlags)0)
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 // Creates a buffer of size |num_bytes| bytes that can be shared between
 // applications (by duplicating the handle -- see |MojoDuplicateBufferHandle()|
 // -- and passing it over a message pipe). To access the buffer, one must call
 // |MojoMapBuffer()|.
 //
 // |options| may be set to null for a shared buffer with the default options.
 //
 // On success, |*shared_buffer_handle| will be set to the handle for the shared
 // buffer. (On failure, it is not modified.)
@@ skipping 88 matching lines @@
 // have been the result of |MojoMapBuffer()| (not some other pointer inside
 // the mapped memory), and the entire mapping will be removed (partial unmapping
 // is not supported). A mapping may only be unmapped once.
 //
 // Returns:
 //   |MOJO_RESULT_OK| on success.
 //   |MOJO_RESULT_INVALID_ARGUMENT| if |buffer| is invalid (e.g., is not the
 //       result of |MojoMapBuffer()| or has already been unmapped).
 MojoResult MojoUnmapBuffer(void* buffer);  // In.
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_BUFFER_H_