De-flake the LibDispatchTaskRunner tests.

base/mac/libdispatch_task_runner.h

 // Copyright (c) 2012 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 BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_
 #define BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_
 
 #include <dispatch/dispatch.h>
 
 #include "base/single_thread_task_runner.h"
+#include "base/synchronization/waitable_event.h"
 
 namespace base {
 namespace mac {
 
 // This is an implementation of the TaskRunner interface that runs closures on
 // a thread managed by Apple's libdispatch. This has the benefit of being able
 // to PostTask() and friends to a dispatch queue, while being reusable as a
 // dispatch_queue_t.
 //
 // One would use this class if an object lives exclusively on one thread but
@@ skipping 20 matching lines @@
                                const Closure& task,
                                base::TimeDelta delay) OVERRIDE;
   virtual bool RunsTasksOnCurrentThread() const OVERRIDE;
 
   // base::SequencedTaskRunner:
   virtual bool PostNonNestableDelayedTask(
       const tracked_objects::Location& from_here,
       const Closure& task,
       base::TimeDelta delay) OVERRIDE;
 
+  // This blocks the calling thread until all work on the dispatch queue has
+  // been run and the queue has been destroyed. Destroying a queue requires
+  // ALL retained references to it to be released. Any new tasks posted to
+  // this thread after shutdown are dropped.
+  void Shutdown();
+
   // Returns the dispatch queue associated with this task runner, for use with
   // system APIs that take dispatch queues. The caller is responsible for
   // retaining the result.
+  //
+  // All properties (context, finalizer, etc.) are managed by this class, and
+  // clients should only use the result of this for dispatch_async().
   dispatch_queue_t GetDispatchQueue() const;
 
  protected:
   virtual ~LibDispatchTaskRunner();
 
  private:
+  static void Finalizer(void* context);
+
   dispatch_queue_t queue_;
+
+  // The event on which Shutdown waits until Finalizer runs.
+  base::WaitableEvent queue_finalized_;
 };
 
 }  // namespace mac
 }  // namespace base
 
 #endif  // BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_