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

mojo/public/c/system/wait.h

Index: mojo/public/c/system/wait.h
diff --git a/mojo/public/c/system/wait.h b/mojo/public/c/system/wait.h
index 4ac513bfda4deebbedc8e98721066d97d7797f63..90d2f9a24f3349b2e571f96124e41cd86934fb9f 100644
--- a/mojo/public/c/system/wait.h
+++ b/mojo/public/c/system/wait.h
@@ -10,12 +10,11 @@
 #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.
@@ -101,8 +100,6 @@ MojoResult MojoWaitMany(
     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_