base: Make it possible to replace the MessageLoop's task runner

base/message_loop/bindable_single_thread_task_runner.h

Index: base/message_loop/bindable_single_thread_task_runner.h
diff --git a/base/message_loop/bindable_single_thread_task_runner.h b/base/message_loop/bindable_single_thread_task_runner.h
new file mode 100644
index 0000000000000000000000000000000000000000..9243dfafd7cd062ac4ff853736d9dd69a61c3316
--- /dev/null
+++ b/base/message_loop/bindable_single_thread_task_runner.h
@@ -0,0 +1,27 @@
+// Copyright 2015 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_MESSAGE_LOOP_BINDABLE_SINGLE_THREAD_TASK_RUNNER_H_
+#define BASE_MESSAGE_LOOP_BINDABLE_SINGLE_THREAD_TASK_RUNNER_H_
+
+#include "base/single_thread_task_runner.h"
+
+namespace base {
+
+// A SingleThreadTaskRunner which can be created on any thread and subsequently
+// be bound to the specific thread it's intended to run tasks on.  Binding is
+// required before any tasks can be posted.
+class BASE_EXPORT BindableSingleThreadTaskRunner
+    : public SingleThreadTaskRunner {
+ public:
+  // Initialize this task runner on the current thread.
+  virtual void BindToCurrentThread() = 0;
+
+ protected:
+  ~BindableSingleThreadTaskRunner() override {}
+};
+
+}  // namespace base
+
+#endif  // BASE_MESSAGE_LOOP_BINDABLE_SINGLE_THREAD_TASK_RUNNER_H_