Long tasks: add a trace event for long tasks

third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc

Index: third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc b/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
index 0108188aa2d157ad84bed4ada7e10efd657fe5e0..95f5d2d481011c0d9974e54b6850c28307b55eb2 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
+++ b/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
@@ -19,6 +19,8 @@
 #include "platform/scheduler/base/work_queue_sets.h"
 #include "platform/wtf/PtrUtil.h"
 
+static const double kLongTaskTraceEventThreshold = 0.05;
+
 namespace blink {
 namespace scheduler {
 
@@ -535,11 +537,11 @@ TaskQueueManager::ProcessTaskResult TaskQueueManager::ProcessTaskFromWorkQueue(
 
   currently_executing_task_queue_ = prev_executing_task_queue;
 
-
+  double task_end_time = 0;
   if (queue->GetShouldNotifyObservers()) {
     if (task_start_time) {
       *time_after_task = real_time_domain()->Now();
-      double task_end_time = MonotonicTimeInSeconds(*time_after_task);
+      task_end_time = MonotonicTimeInSeconds(*time_after_task);
 
       queue->OnTaskCompleted(
           base::TimeTicks() + base::TimeDelta::FromSecondsD(task_start_time),
@@ -554,6 +556,12 @@ TaskQueueManager::ProcessTaskResult TaskQueueManager::ProcessTaskFromWorkQueue(
     queue->NotifyDidProcessTask(pending_task);
   }
 
+  if (task_start_time && task_end_time &&
+      task_end_time - task_start_time > kLongTaskTraceEventThreshold) {
+    TRACE_EVENT_INSTANT1("blink", "LongTask", TRACE_EVENT_SCOPE_THREAD,
+                         "duration", task_end_time - task_start_time);
+  }
+
   return ProcessTaskResult::EXECUTED;
 }