Introduce the concept of media task runner.

chromecast/media/cma/base/media_task_runner.h

+// Copyright 2014 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 CHROMECAST_MEDIA_CMA_BASE_MEDIA_TASK_RUNNER_H_
+#define CHROMECAST_MEDIA_CMA_BASE_MEDIA_TASK_RUNNER_H_
+
+#include "base/callback.h"
+#include "base/location.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/time/time.h"
+
+namespace chromecast {
+namespace media {
+
+class MediaTaskRunner
+    : public base::RefCountedThreadSafe<MediaTaskRunner> {
+ public:
+  MediaTaskRunner();
+
+  // Post a task with the given media |timestamp|. If |timestamp| is equal to
+  // |kNoTimestamp()|, the task is scheduled right away.
+  // Returns true if the task may be run at some point in the future, and false
+  // if the task definitely will not be run.
+  virtual bool PostMediaTask(
+      const tracked_objects::Location& from_here,
+      const base::Closure& task,
+      base::TimeDelta timestamp) = 0;

[xhwang, 2014/09/03 17:06:39]

How is media |timestamp| used to calculate the time when the task will actually
be run? I can understand it with the context of the balanced task runner. But it
seems a bit odd to look at this as a standalong interface. Since we have only
one impl of this interface, can we just use the concrete class without having
this interface?

[damienv1, 2014/09/03 22:01:03]

1) Note added about how |timestamp| should be used.

2) Agree that currently all the media task runners could be "hard-coded" without
using a base class.
   However, I wanted to make it similar to the base::TaskRunner design
   and I was planning initially to use the same interface for synchronization (a
derived class using a Clock could be used to synchronized audio with video and
that would fix some of the TODOs we have in the current video_renderer_impl).
   So I'd like to keep it for now.

+
+ protected:
+  virtual ~MediaTaskRunner();
+  friend class base::RefCountedThreadSafe<MediaTaskRunner>;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MediaTaskRunner);
+};
+
+}  // namespace media
+}  // namespace chromecast
+
+#endif  // CHROMECAST_MEDIA_CMA_BASE_MEDIA_TASK_RUNNER_H_