[TaskScheduler]: Add worker alive trace event.

This CL adds trace event "SchedulerWorkerThread alive" to track lifespan of a
SchedulerWorker.

Change-Id: Id1a6be3a9235a0522c7618918c3453423db153e3
Reviewed-on: https://chromium-review.googlesource.com/1172925Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584229}

base/task/task_scheduler/scheduler_worker.cc

@@ -283,6 +283,8 @@ NOINLINE void SchedulerWorker::RunBackgroundDedicatedCOMWorker() {
 
 void SchedulerWorker::RunWorker() {
   DCHECK_EQ(self_, this);
+  TRACE_EVENT_BEGIN0(TRACE_DISABLED_BY_DEFAULT("task_scheduler_diagnostics"),
+                     "SchedulerWorkerThread alive");
   TRACE_EVENT_BEGIN0("task_scheduler", "SchedulerWorkerThread active");
 
   if (scheduler_worker_observer_)
@@ -356,6 +358,8 @@ void SchedulerWorker::RunWorker() {
   self_ = nullptr;
 
   TRACE_EVENT_END0("task_scheduler", "SchedulerWorkerThread active");
+  TRACE_EVENT_END0(TRACE_DISABLED_BY_DEFAULT("task_scheduler_diagnostics"),
+                   "SchedulerWorkerThread alive");
 }
 
 }  // namespace internal