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

mojo/public/c/system/message_pipe.h

Index: mojo/public/c/system/message_pipe.h
diff --git a/mojo/public/c/system/message_pipe.h b/mojo/public/c/system/message_pipe.h
index 5f68b79c2cec7fe85055dd11fe739d17037f6eb1..3722814119d3580abaa3ee362b095e555e61807c 100644
--- a/mojo/public/c/system/message_pipe.h
+++ b/mojo/public/c/system/message_pipe.h
@@ -54,9 +54,7 @@ typedef uint32_t MojoReadMessageFlags;
 #define MOJO_READ_MESSAGE_FLAG_NONE ((MojoReadMessageFlags)0)
 #define MOJO_READ_MESSAGE_FLAG_MAY_DISCARD ((MojoReadMessageFlags)1 << 0)
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 // Creates a message pipe, which is a bidirectional communication channel for
 // framed data (i.e., messages). Messages can contain plain data and/or Mojo
@@ -151,8 +149,6 @@ MojoResult MojoReadMessage(MojoHandle message_pipe_handle,
                            uint32_t* num_handles,  // Optional in/out.
                            MojoReadMessageFlags flags);
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_MESSAGE_PIPE_H_