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

mojo/public/c/system/data_pipe.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 data pipes.
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_DATA_PIPE_H_
 #define MOJO_PUBLIC_C_SYSTEM_DATA_PIPE_H_
@@ skipping 60 matching lines @@
 //       |MOJO_READ_DATA_FLAG_DISCARD| and |MOJO_READ_DATA_FLAG_QUERY|.
 
 typedef uint32_t MojoReadDataFlags;
 
 #define MOJO_READ_DATA_FLAG_NONE ((MojoReadDataFlags)0)
 #define MOJO_READ_DATA_FLAG_ALL_OR_NONE ((MojoReadDataFlags)1 << 0)
 #define MOJO_READ_DATA_FLAG_DISCARD ((MojoReadDataFlags)1 << 1)
 #define MOJO_READ_DATA_FLAG_QUERY ((MojoReadDataFlags)1 << 2)
 #define MOJO_READ_DATA_FLAG_PEEK ((MojoReadDataFlags)1 << 3)
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 // Creates a data pipe, which is a unidirectional communication channel for
 // unframed data, with the given options. Data is unframed, but must come as
 // (multiples of) discrete elements, of the size given in |options|. See
 // |MojoCreateDataPipeOptions| for a description of the different options
 // available for data pipes.
 //
 // |options| may be set to null for a data pipe with the default options (which
 // will have an element size of one byte and have some system-dependent
 // capacity).
@@ skipping 200 matching lines @@
 //   |MOJO_RESULT_INVALID_ARGUMENT| if some argument was invalid (e.g.,
 //       |data_pipe_consumer_handle| is not a handle to a data pipe consumer or
 //       |num_bytes_written| is greater than the maximum value provided by
 //       |MojoBeginReadData()| or not a multiple of the element size).
 //   |MOJO_RESULT_FAILED_PRECONDITION| if the data pipe consumer is not in a
 //       two-phase read (e.g., |MojoBeginReadData()| was not called or
 //       |MojoEndReadData()| has already been called).
 MojoResult MojoEndReadData(MojoHandle data_pipe_consumer_handle,
                            uint32_t num_bytes_read);
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_DATA_PIPE_H_