Redirect the MessageLoop's task runner to the renderer scheduler

components/scheduler/child/task_queue_manager.cc

 // Copyright 2014 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.
 
 #include "components/scheduler/child/task_queue_manager.h"
 
 #include <queue>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 492 matching lines @@
   std::vector<const base::TaskQueue*> work_queues;
   for (const auto& queue : queues_)
     work_queues.push_back(&queue->work_queue());
   selector_->RegisterWorkQueues(work_queues);
   selector_->SetTaskQueueSelectorObserver(this);
 
   do_work_from_main_thread_closure_ =
       base::Bind(&TaskQueueManager::DoWork, weak_factory_.GetWeakPtr(), true);
   do_work_from_other_thread_closure_ =
       base::Bind(&TaskQueueManager::DoWork, weak_factory_.GetWeakPtr(), false);
+
+  if (task_queue_count)
+    main_task_runner->SetDefaultTaskRunner(Queue(0));

[alex clarke (OOO till 29th), 2015/07/08 13:30:53]

It's currently true that task #0 is the default queue.  Do we want to formalize
that somehow, or to pass the default task queue index in as a parameter?

[Sami, 2015/07/08 18:45:07]

Good point. I thought about this a bit an realized that the task queue manager
doesn't really even need to know about the default task runner -- it's more of a
helper thing. Because of this I've now added a new interface called
SchedulerTaskRunnerDelegate which basically adds the default task runner
overriding on top of NestableSingleThreadTaskRunner. Unfortunately this makes
the patch a little bigger, but I think the refactoring makes sense especially if
we want to move TQM to base some day.

 }
 
 TaskQueueManager::~TaskQueueManager() {
   TRACE_EVENT_OBJECT_DELETED_WITH_ID(disabled_by_default_tracing_category_,
                                      "TaskQueueManager", this);
   for (auto& queue : queues_)
     queue->WillDeleteTaskQueueManager();
   selector_->SetTaskQueueSelectorObserver(nullptr);
 }
 
@@ skipping 279 matching lines @@
       return nullptr;
   }
 }
 
 void TaskQueueManager::OnTaskQueueEnabled() {
   DCHECK(main_thread_checker_.CalledOnValidThread());
   MaybePostDoWorkOnMainRunner();
 }
 
 }  // namespace scheduler