Mojo: Update/add README.md files.

mojo/public/c/environment/async_waiter.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.
 
-#ifndef MOJO_PUBLIC_C_SYSTEM_ASYNC_WAITER_H_
-#define MOJO_PUBLIC_C_SYSTEM_ASYNC_WAITER_H_
+#ifndef MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_
+#define MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_
 
 #include "mojo/public/c/system/core.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 typedef uintptr_t MojoAsyncWaitID;
 
 typedef void (*MojoAsyncWaitCallback)(void* closure, MojoResult result);
@@ skipping 13 matching lines @@
   // Cancel an existing call to AsyncWait with the given MojoAsyncWaitID. The
   // corresponding MojoAsyncWaitCallback will not be called in this case.
   void (*CancelWait)(struct MojoAsyncWaiter* waiter,
                      MojoAsyncWaitID wait_id);
 };
 
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
-#endif  // MOJO_PUBLIC_C_SYSTEM_ASYNC_WAITER_H_
+#endif  // MOJO_PUBLIC_C_ENVIRONMENT_ASYNC_WAITER_H_