Add support to base::Timer for custom task runners.

base/timer/timer.h

Index: base/timer/timer.h
diff --git a/base/timer/timer.h b/base/timer/timer.h
index 4ef2f456395d5f4b44ebaedbcb06145f6c30cf5f..7e2c1d41f134aa7bdc1c950c01c7645a782cc184 100644
--- a/base/timer/timer.h
+++ b/base/timer/timer.h
@@ -60,6 +60,7 @@
 namespace base {
 
 class BaseTimerTaskInternal;
+class SingleThreadTaskRunner;
 
 //-----------------------------------------------------------------------------
 // This class wraps MessageLoop::PostDelayedTask to manage delayed and repeating
@@ -87,6 +88,10 @@ class BASE_EXPORT Timer {
   // Returns the current delay for this timer.
   virtual TimeDelta GetCurrentDelay() const;
 
+  // Set the task runner on which the task should be scheduled. This method can
+  // only be called before any tasks have been scheduled.
+  virtual void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
+
   // Start the timer to run at the given |delay| from now. If the timer is
   // already running, it will be replaced to call the given |user_task|.
   virtual void Start(const tracked_objects::Location& posted_from,
@@ -128,6 +133,11 @@ class BASE_EXPORT Timer {
   // and desired_run_time_ are reset to Now() + delay.
   void PostNewScheduledTask(TimeDelta delay);
 
+  // Returns the task runner on which the task should be scheduled. If the
+  // corresponding task_runner_ field is null, the task runner for the current
+  // thread is returned.
+  scoped_refptr<SingleThreadTaskRunner> GetTaskRunner();
+
   // Disable scheduled_task_ and abandon it so that it no longer refers back to
   // this object.
   void AbandonScheduledTask();
@@ -145,6 +155,10 @@ class BASE_EXPORT Timer {
   // RunScheduledTask() at scheduled_run_time_.
   BaseTimerTaskInternal* scheduled_task_;
 
+  // The task runner on which the task should be scheduled. If it is null, the
+  // task runner for the current thread should be used.
+  scoped_refptr<SingleThreadTaskRunner> task_runner_;
+
   // Location in user code.
   tracked_objects::Location posted_from_;
   // Delay requested by user.