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

mojo/public/cpp/system/watcher.h

Index: mojo/public/cpp/system/watcher.h
diff --git a/mojo/public/cpp/system/watcher.h b/mojo/public/cpp/system/watcher.h
index 815b73b3b50b1d7dc71bbb465587862a2aeb2301..3284154b2d886d97b9ecbde4e23e2255ce121077 100644
--- a/mojo/public/cpp/system/watcher.h
+++ b/mojo/public/cpp/system/watcher.h
@@ -4,6 +4,8 @@
 
 #ifndef MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
+
+#include <memory>
 
 #include "base/callback.h"
 #include "base/macros.h"
@@ -34,6 +36,11 @@
   //
   //   |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 =
@@ -75,6 +82,9 @@
   ReadyCallback ready_callback() const { return callback_; }
 
  private:
+  class MessageLoopObserver;
+  friend class MessageLoopObserver;
+
   void OnHandleReady(MojoResult result);
 
   static void CallOnHandleReady(uintptr_t context,
@@ -90,6 +100,8 @@
   // 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