TaskScheduler: Don't use a callback to post a task from TaskTracker.

base/task_scheduler/task_tracker.cc

Index: base/task_scheduler/task_tracker.cc
diff --git a/base/task_scheduler/task_tracker.cc b/base/task_scheduler/task_tracker.cc
index 50922bbc8cf9004491bc59c1ac4d6bdbf3e1c5d3..c896a9fad9b4f517f0f78499d4df31052668a14a 100644
--- a/base/task_scheduler/task_tracker.cc
+++ b/base/task_scheduler/task_tracker.cc
@@ -58,19 +58,16 @@ void TaskTracker::Shutdown() {
   }
 }
 
-void TaskTracker::PostTask(
-    const Callback<void(std::unique_ptr<Task>)>& post_task_callback,
-    std::unique_ptr<Task> task) {
-  DCHECK(!post_task_callback.is_null());
+bool TaskTracker::WillPostTask(const Task* task) {
   DCHECK(task);
 
   if (!BeforePostTask(task->traits.shutdown_behavior()))
-    return;
+    return false;
 
   debug::TaskAnnotator task_annotator;
   task_annotator.DidQueueTask(kQueueFunctionName, *task);

[gab, 2016/04/07 17:05:18]

Not sure how big of a refactor that would be but it really feels like this
should also be WillPostTask or WillQueueTask, it feels wrong to call
DidQueueTask from WillPostTask... (not something for this CL but...).

[fdoray, 2016/04/07 18:05:58]

It's used at 4 other places. Sometimes it's called before the task is queued,
sometimes it's called after... It should be easy to rename it and always call it
before queuing the task.

 
-  post_task_callback.Run(std::move(task));
+  return true;
 }
 
 void TaskTracker::RunTask(const Task* task) {

[gab, 2016/04/07 17:05:17]

I think I'd asked that on the initial TaskTracker CL and forget the answer, but
shouldn't this take a std::uniqueptr<Task>? Why would the caller want to retain
ownership?

[fdoray, 2016/04/07 18:05:58]

The Task is owned by its Sequence when it runs.

If we wanted to be sure that a Task doesn't run twice, we could have
TaskTracker::RunTask(scoped_refptr<Sequence> sequence) which runs the next task
in |sequence| and pops it (I would prefer no to add a TaskTracker->Sequence
dependency).

[gab, 2016/04/07 18:58:57]

Ah right, ok let's keep it this way then.