Revert of Remove MessageLoop::current() from base::win::ObjectWatcher.

base/win/object_watcher.h

Index: base/win/object_watcher.h
diff --git a/base/win/object_watcher.h b/base/win/object_watcher.h
index a2821c114f18279a5cc4c9b57d77d23cfd35439f..e1e86fe73aa38e421ef40eaaead7d550bf14812f 100644
--- a/base/win/object_watcher.h
+++ b/base/win/object_watcher.h
@@ -10,9 +10,8 @@
 #include "base/base_export.h"
 #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/message_loop/message_loop.h"
 
 namespace base {
 namespace win {
@@ -47,22 +46,22 @@
 // If the object is already signaled before being watched, OnObjectSignaled is
 // still called after (but not necessarily immediately after) watch is started.
 //
-// NOTE: Except for the constructor, all public methods of this class must be
-// called on the same thread. A ThreadTaskRunnerHandle must be set on that
-// thread.
-class BASE_EXPORT ObjectWatcher {
+// NOTE: Use of this class requires that there be a current message loop;
+// otherwise, when the object is signaled, there would be no loop to post the
+// callback task to.  This means that you cannot use ObjectWatcher in test code
+// that doesn't create a message loop (unless you add such a loop).
+class BASE_EXPORT ObjectWatcher : public MessageLoop::DestructionObserver {
  public:
   class BASE_EXPORT Delegate {
    public:
     virtual ~Delegate() {}
-    // Called from the thread that started the watch when a signaled object is
-    // detected. To continue watching the object, StartWatching must be called
-    // again.
+    // Called from the MessageLoop when a signaled object is detected.  To
+    // continue watching the object, StartWatching must be called again.
     virtual void OnObjectSignaled(HANDLE object) = 0;
   };
 
   ObjectWatcher();
-  ~ObjectWatcher();
+  ~ObjectWatcher() override;
 
   // When the object is signaled, the given delegate is notified on the thread
   // where StartWatchingOnce is called. The ObjectWatcher is not responsible for
@@ -100,23 +99,15 @@
 
   void Signal(Delegate* delegate);
 
-  void Reset();
+  // MessageLoop::DestructionObserver implementation:
+  void WillDestroyCurrentMessageLoop() override;
 
-  // A callback pre-bound to Signal() that is posted to the caller's task runner
-  // when the wait completes.
+  // Internal state.
   Closure callback_;
-
-  // The object being watched.
-  HANDLE object_ = nullptr;
-
-  // The wait handle returned by RegisterWaitForSingleObject.
-  HANDLE wait_object_ = nullptr;
-
-  // The task runner of the thread on which the watch was started.
-  scoped_refptr<SingleThreadTaskRunner> task_runner_;
-
-  bool run_once_ = true;
-
+  HANDLE object_;             // The object being watched
+  HANDLE wait_object_;        // Returned by RegisterWaitForSingleObject
+  MessageLoop* origin_loop_;  // Used to get back to the origin thread
+  bool run_once_;
   WeakPtrFactory<ObjectWatcher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);