Reland "[scheduler] Add task type and queue type to tracing."

This is a reland of 775612b8

Original change's description:
> [scheduler] Add task type and queue type to tracing.
> 
> R=skyostil@chromium.org
> 
> Change-Id: Id0aea377558e876c8178e82eb3a170f678c415cc
> Reviewed-on: https://chromium-review.googlesource.com/1140717
> Commit-Queue: Alexander Timin <altimin@chromium.org>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#576201}

Change-Id: I8289cc417637e7e605dbb5f9df8cc6ea68fa457f
Reviewed-on: https://chromium-review.googlesource.com/1143224Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577159}

base/task/sequence_manager/sequence_manager_impl.cc

@@ -293,6 +293,24 @@ void SequenceManagerImpl::SetNextDelayedDoWork(LazyNow* lazy_now,
 }
 
 Optional<PendingTask> SequenceManagerImpl::TakeTask() {
+  Optional<PendingTask> task = TakeTaskImpl();
+  if (!task)
+    return base::nullopt;
+
+  ExecutingTask& executing_task =
+      *main_thread_only().task_execution_stack.rbegin();
+
+  // It's important that there are no active trace events here which will
+  // terminate before we finish executing the task.
+  TRACE_EVENT_BEGIN2(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
+                     "SequenceManager::RunTask", "queue_type",
+                     executing_task.task_queue->GetName(), "task_type",
+                     executing_task.task_type);
+
+  return task;
+}
+
+Optional<PendingTask> SequenceManagerImpl::TakeTaskImpl() {
   CHECK(Validate());
 
   DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);
@@ -357,6 +375,7 @@ Optional<PendingTask> SequenceManagerImpl::TakeTask() {
     ExecutingTask& executing_task =
         *main_thread_only().task_execution_stack.rbegin();
     NotifyWillProcessTask(&executing_task, &lazy_now);
+
     return std::move(executing_task.pending_task);
   }
 }
@@ -365,6 +384,10 @@ void SequenceManagerImpl::DidRunTask() {
   LazyNow lazy_now(controller_->GetClock());
   ExecutingTask& executing_task =
       *main_thread_only().task_execution_stack.rbegin();
+
+  TRACE_EVENT_END0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
+                   "SequenceManagerImpl::RunTask");
+
   NotifyDidProcessTask(&executing_task, &lazy_now);
   main_thread_only().task_execution_stack.pop_back();
 

base/task/sequence_manager/sequence_manager_impl.h

@@ -171,16 +171,20 @@ class BASE_EXPORT SequenceManagerImpl
   // selector interface is unaware of those.  This struct keeps track off all
   // task related state needed to make pairs of TakeTask() / DidRunTask() work.
   struct ExecutingTask {
-    ExecutingTask(internal::TaskQueueImpl::Task&& pending_task,
+    ExecutingTask(internal::TaskQueueImpl::Task&& task,
                   internal::TaskQueueImpl* task_queue,
                   TaskQueue::TaskTiming task_timing)
-        : pending_task(std::move(pending_task)),
+        : pending_task(std::move(task)),
           task_queue(task_queue),
-          task_timing(task_timing) {}
+          task_timing(task_timing),
+          task_type(pending_task.task_type()) {}
 
     internal::TaskQueueImpl::Task pending_task;
     internal::TaskQueueImpl* task_queue = nullptr;
     TaskQueue::TaskTiming task_timing;
+    // Save task metadata to use in after running a task as |pending_task|
+    // won't be available then.
+    int task_type;
   };
 
   struct MainThreadOnly {
@@ -286,6 +290,10 @@ class BASE_EXPORT SequenceManagerImpl
 
   bool ShouldRecordCPUTimeForTask();
 
+  // Helper to terminate all scoped trace events to allow starting new ones
+  // in TakeTask().
+  Optional<PendingTask> TakeTaskImpl();
+
   // Determines if wall time or thread time should be recorded for the next
   // task.
   TaskQueue::TaskTiming InitializeTaskTiming(