Change the canonical way to include the C macros.h to <mojo/macros.h>.

mojo/public/c/system/tests/compile_unittest_pure_cpp.cc

 // 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 is used to ensure that our C headers can be compiled as C++ with
 // more stringent warnings, in particular with "-Wundef".
 
 // Include all the header files that are meant to be compilable as C.
+#include <mojo/macros.h>
+
+// Include all the header files that are meant to be compilable as C.
 #include "mojo/public/c/environment/async_waiter.h"
 #include "mojo/public/c/environment/logger.h"
 #include "mojo/public/c/system/buffer.h"
 #include "mojo/public/c/system/data_pipe.h"
 #include "mojo/public/c/system/handle.h"
-#include "mojo/public/c/system/macros.h"
 #include "mojo/public/c/system/main.h"
 #include "mojo/public/c/system/message_pipe.h"
 #include "mojo/public/c/system/result.h"
 #include "mojo/public/c/system/time.h"
 #include "mojo/public/c/system/wait.h"
 #include "mojo/public/c/system/wait_set.h"
 
 // We don't actually want to test anything; we just want to make sure that this
 // file is compiled/linked in (this function is called from core_unittest.cc).
 const char* MinimalCppTest() {
   return nullptr;
 }