Record PendingTaskCount when a backgrounded renderer is suspended.

third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc

Index: third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc b/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
index 0d5d624c3473fbf9c57ddb54950c5c75b4a7da83..44f0e5bf770884255e1313cda63b0b829d2ecb8e 100644
--- a/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
+++ b/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
@@ -490,12 +490,28 @@ void RendererSchedulerImpl::SuspendRenderer() {
   if (helper_.IsShutdown())
     return;
   suspend_timers_when_backgrounded_closure_.Cancel();
+
+  UMA_HISTOGRAM_COUNTS("PurgeAndSuspend.PendingTaskCount",
+                       GetNumberOfPendingTasks());
+
   // TODO(hajimehoshi): We might need to suspend not only timer queue but also
   // e.g. loading tasks or postMessage.
   MainThreadOnly().renderer_suspended = true;
   SuspendTimerQueueWhenBackgrounded();
 }
 
+int RendererSchedulerImpl::GetNumberOfPendingTasks() {

[Sami, 2016/10/14 01:38:18]

I would suggest adding this getter into TaskQueueManager instead so it can just
iterate over |queues_| there. Otherwise it is easy to forget to add a queue here
when we introduce new ones.

[tasak, 2016/10/14 05:55:32]

I see. I agree that TaskQueueManager is the best class to add the getter.
Done.

+  int count = 0;
+  for (const scoped_refptr<TaskQueue>& loading_queue : loading_task_runners_)
+    count += loading_queue->GetNumberOfPendingTasks();
+  for (const scoped_refptr<TaskQueue>& timer_queue : timer_task_runners_)
+    count += timer_queue->GetNumberOfPendingTasks();
+  count += DefaultTaskRunner()->GetNumberOfPendingTasks();
+  count += CompositorTaskRunner()->GetNumberOfPendingTasks();
+  count += idle_helper_.GetNumberOfPendingIdleTasks();
+  return count;
+}
+
 void RendererSchedulerImpl::EndIdlePeriod() {
   if (MainThreadOnly().in_idle_period_for_testing)
     return;