Mojo: Update/add README.md files.

mojo/public/c/system/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_
-
-#include "mojo/public/c/system/core.h"
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-typedef uintptr_t MojoAsyncWaitID;
-
-typedef void (*MojoAsyncWaitCallback)(void* closure, MojoResult result);
-
-struct MojoAsyncWaiter {
-  // Asynchronously call MojoWait on a background thread, and pass the result
-  // of MojoWait to the given MojoAsyncWaitCallback on the current thread.
-  // Returns a non-zero MojoAsyncWaitID that can be used with CancelWait to
-  // stop waiting. This identifier becomes invalid once the callback runs.
-  MojoAsyncWaitID (*AsyncWait)(struct MojoAsyncWaiter* waiter,
-                               MojoHandle handle,
-                               MojoWaitFlags flags,
-                               MojoDeadline deadline,
-                               MojoAsyncWaitCallback callback,
-                               void* closure);
-
-  // 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_