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..111e3b6c767760f0eff067cdc099353cedce6cb2
--- /dev/null
+++ b/base/message_loop/bindable_single_thread_task_runner.h
@@ -0,0 +1,26 @@
+// 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 bound to a specific thread after it

[danakj, 2015/06/29 19:07:23]

what does "can be bound to a specific thread" here mean. Does it mean "must be
bound to a thread before being used, and only used on that thread"? Does it mean
"optionally may be bound to a thread" (then what if it isn't)? Please add some
mode details here.

[alex clarke (OOO till 29th), 2015/06/30 10:51:43]

Done.

+// has been constructed.
+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_