Revert of Remove MessageLoop destruction observer from mojo::Watcher.

mojo/public/cpp/system/watcher.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.
 
 #ifndef MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
 
+#include <memory>
+
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_checker.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "mojo/public/c/system/types.h"
 #include "mojo/public/cpp/system/handle.h"
 #include "mojo/public/cpp/system/system_export.h"
 
 namespace mojo {
 
 // A Watcher watches a single Mojo handle for signal state changes.
 //
 // NOTE: Watchers may only be used on threads which have a running MessageLoop.
 class MOJO_CPP_SYSTEM_EXPORT Watcher {
  public:
   // A callback to be called any time a watched handle changes state in some
   // interesting way. The |result| argument indicates one of the following
   // conditions depending on its value:
   //
   //   |MOJO_RESULT_OK|: One or more of the signals being watched is satisfied.
   //
   //   |MOJO_RESULT_FAILED_PRECONDITION|: None of the signals being watched can
   //       ever be satisfied again.
   //
   //   |MOJO_RESULT_CANCELLED|: The handle has been closed and the watch has
   //       been cancelled implicitly.
+  //
+  //   |MOJO_RESULT_ABORTED|: Notifications can no longer be delivered for this
+  //       watcher for some unspecified reason, e.g., the watching thread may
+  //       be shutting down soon. Note that it is still necessary to explicitly
+  //       Cancel() the watch in this case.
   using ReadyCallback = base::Callback<void(MojoResult result)>;
 
   explicit Watcher(scoped_refptr<base::SingleThreadTaskRunner> runner =
                        base::ThreadTaskRunnerHandle::Get());
 
   // NOTE: This destructor automatically calls |Cancel()| if the Watcher is
   // still active.
   ~Watcher();
 
   // Indicates if the Watcher is currently watching a handle.
@@ skipping 21 matching lines @@
                    const ReadyCallback& callback);
 
   // Cancels the current watch. Once this returns, the callback previously
   // passed to |Start()| will never be called again for this Watcher.
   void Cancel();
 
   Handle handle() const { return handle_; }
   ReadyCallback ready_callback() const { return callback_; }
 
  private:
+  class MessageLoopObserver;
+  friend class MessageLoopObserver;
+
   void OnHandleReady(MojoResult result);
 
   static void CallOnHandleReady(uintptr_t context,
                                 MojoResult result,
                                 MojoHandleSignalsState signals_state,
                                 MojoWatchNotificationFlags flags);
 
   base::ThreadChecker thread_checker_;
 
   // The TaskRunner of this Watcher's owning thread. This field is safe to
   // access from any thread.
   const scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
   // Whether |task_runner_| is the same as base::ThreadTaskRunnerHandle::Get()
   // for the thread.
   const bool is_default_task_runner_;
 
+  std::unique_ptr<MessageLoopObserver> message_loop_observer_;
+
   // A persistent weak reference to this Watcher which can be passed to the
   // Dispatcher any time this object should be signalled. Safe to access (but
   // not to dereference!) from any thread.
   base::WeakPtr<Watcher> weak_self_;
 
   // Fields below must only be accessed on the Watcher's owning thread.
 
   // The handle currently under watch. Not owned.
   Handle handle_;
 
   // The callback to call when the handle is signaled.
   ReadyCallback callback_;
 
   base::WeakPtrFactory<Watcher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Watcher);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_