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 3284154b2d886d97b9ecbde4e23e2255ce121077..815b73b3b50b1d7dc71bbb465587862a2aeb2301 100644
--- a/mojo/public/cpp/system/watcher.h
+++ b/mojo/public/cpp/system/watcher.h
@@ -5,8 +5,6 @@
 #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"
@@ -36,11 +34,6 @@ class MOJO_CPP_SYSTEM_EXPORT Watcher {
   //
   //   |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 =
@@ -82,9 +75,6 @@ class MOJO_CPP_SYSTEM_EXPORT Watcher {
   ReadyCallback ready_callback() const { return callback_; }
 
  private:
-  class MessageLoopObserver;
-  friend class MessageLoopObserver;
-
   void OnHandleReady(MojoResult result);
 
   static void CallOnHandleReady(uintptr_t context,
@@ -101,8 +91,6 @@ class MOJO_CPP_SYSTEM_EXPORT Watcher {
   // 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.