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 6d282ee8e7ac84f3d6f5757650c08be70463f9e6..c9e9050e1f51416702f74e992b626cee0c32cda2 100644
--- a/base/timer/timer.h
+++ b/base/timer/timer.h
@@ -55,6 +55,7 @@
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/location.h"
+#include "base/single_thread_task_runner.h"
 #include "base/time/time.h"
 
 namespace base {
@@ -87,6 +88,9 @@ 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.

[Sami, 2014/10/13 17:00:11]

Please mention that this can only be changed before any tasks have been
scheduled.

[petrcermak, 2014/10/13 17:21:40]

Done.

+  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,
@@ -122,6 +126,11 @@ 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> ResolveTaskRunner() const;
+
   // Disable scheduled_task_ and abandon it so that it no longer refers back to
   // this object.
   void AbandonScheduledTask();
@@ -139,6 +148,10 @@ class BASE_EXPORT Timer {
   // RunScheduledTask() at scheduled_run_time_.
   BaseTimerTaskInternal* scheduled_task_;
 
+  // The task runner on which the task should be scheduled. If null, the
+  // task runner for the current thread should be used.
+  scoped_refptr<SingleThreadTaskRunner> task_runner_;

[Sami, 2014/10/13 17:00:10]

Could we initialize this to be ThreadTaskRunnerHandle::Get() in the constructor
so we can avoid the dynamic resolving? They're the same types.

[petrcermak, 2014/10/13 17:21:40]

As discussed, this cannot be done in the constructor because the task runner
might not be constructed yet (see the TimerTest.MessageLoopShutdown unit test).
Instead, we initialize it upon the first GetTaskRunner call.

+
   // Location in user code.
   tracked_objects::Location posted_from_;
   // Delay requested by user.