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

mojo/public/c/system/time.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 time-related declarations/definitions.
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_TIME_H_
 #define MOJO_PUBLIC_C_SYSTEM_TIME_H_
 
 #include <stdint.h>
 
+#include "mojo/public/c/system/macros.h"
+
 // |MojoTimeTicks|: A time delta, in microseconds, the meaning of which is
 // source-dependent.
 
 typedef int64_t MojoTimeTicks;
 
 // |MojoDeadline|: Used to specify deadlines (timeouts), in microseconds (except
 // for |MOJO_DEADLINE_INDEFINITE|).
 //   |MOJO_DEADLINE_INDEFINITE| - Used to indicate "forever".
 
 typedef uint64_t MojoDeadline;
 
 #define MOJO_DEADLINE_INDEFINITE ((MojoDeadline)-1)
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 // Note: Pointer parameters that are labelled "optional" may be null (at least
 // under some circumstances). Non-const pointer parameters are also labeled
 // "in", "out", or "in/out", to indicate how they are used. (Note that how/if
 // such a parameter is used may depend on other parameters or the requested
 // operation's success/failure. E.g., a separate |flags| parameter may control
 // whether a given "in/out" parameter is used for input, output, or both.)
 
 // Returns the time, in microseconds, since some undefined point in the past.
 // The values are only meaningful relative to other values that were obtained
 // from the same device without an intervening system restart. Such values are
 // guaranteed to be monotonically non-decreasing with the passage of real time.
 // Although the units are microseconds, the resolution of the clock may vary and
 // is typically in the range of ~1-15 ms.
 MojoTimeTicks MojoGetTimeTicksNow(void);
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_TIME_H_