scheduler: Implement task observers

content/renderer/scheduler/task_queue_manager.cc

Index: content/renderer/scheduler/task_queue_manager.cc
diff --git a/content/renderer/scheduler/task_queue_manager.cc b/content/renderer/scheduler/task_queue_manager.cc
index 112a37ca33c120b61319a12d587106335badfaf0..a50b6097b0b3eea97a998b5c47866a36ed14aa7b 100644
--- a/content/renderer/scheduler/task_queue_manager.cc
+++ b/content/renderer/scheduler/task_queue_manager.cc
@@ -7,6 +7,7 @@
 #include <queue>
 
 #include "base/bind.h"
+#include "base/observer_list.h"
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "cc/test/test_now_source.h"
@@ -413,8 +414,12 @@ void TaskQueueManager::ProcessTaskFromWorkQueue(size_t queue_index) {
     main_task_runner_->PostNonNestableTask(pending_task.posted_from,
                                            pending_task.task);
   } else {
+    FOR_EACH_OBSERVER(base::MessageLoop::TaskObserver, task_observers_,

[alex clarke (OOO till 29th), 2015/02/13 11:02:07]

Aren't we risking a scenario like this:

base::MessageLoop::RunTask
FOR_EACH_OBSERVER(...)
task_annotator_.RunTask()
    ...
    TaskQueueManager::ProcessTaskFromWorkQueue
    FOR_EACH_OBSERVER(...)
    task_annotator_.RunTask()
        actual task runs
    FOR_EACH_OBSERVER(...)

    x 1-4
FOR_EACH_OBSERVER(...)

[Sami, 2015/02/13 12:09:09]

We talked about this offline. It should be fine because we're intercepting all
observers from Blink and those should be the only ones that matter w.r.t.
microtasks.

+                      WillProcessTask(pending_task));
     task_annotator_.RunTask("TaskQueueManager::PostTask",
                             "TaskQueueManager::RunTask", pending_task);
+    FOR_EACH_OBSERVER(base::MessageLoop::TaskObserver, task_observers_,
+                      DidProcessTask(pending_task));
   }
 }
 
@@ -442,6 +447,18 @@ void TaskQueueManager::SetWorkBatchSize(int work_batch_size) {
   work_batch_size_ = work_batch_size;
 }
 
+void TaskQueueManager::AddTaskObserver(
+    base::MessageLoop::TaskObserver* task_observer) {
+  DCHECK(main_thread_checker_.CalledOnValidThread());
+  task_observers_.AddObserver(task_observer);
+}
+
+void TaskQueueManager::RemoveTaskObserver(
+    base::MessageLoop::TaskObserver* task_observer) {
+  DCHECK(main_thread_checker_.CalledOnValidThread());
+  task_observers_.RemoveObserver(task_observer);
+}
+
 void TaskQueueManager::SetTimeSourceForTesting(
     scoped_refptr<cc::TestNowSource> time_source) {
   DCHECK(main_thread_checker_.CalledOnValidThread());