Measure how many tasks are cancelled in MessageLoop.

base/debug/task_annotator.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/debug/task_annotator.h"
 
 #include "base/debug/activity_tracker.h"
 #include "base/debug/alias.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/pending_task.h"
 #include "base/trace_event/trace_event.h"
 #include "base/tracked_objects.h"
 
 namespace base {
 namespace debug {
 
 TaskAnnotator::TaskAnnotator() {
 }
 
 TaskAnnotator::~TaskAnnotator() {
 }
 
 void TaskAnnotator::DidQueueTask(const char* queue_function,
                                  const PendingTask& pending_task) {
   TRACE_EVENT_WITH_FLOW0(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
                           queue_function,
                           TRACE_ID_MANGLE(GetTaskTraceID(pending_task)),
                           TRACE_EVENT_FLAG_FLOW_OUT);
 }
 
 void TaskAnnotator::RunTask(const char* queue_function,
                             const PendingTask& pending_task) {
+  UMA_HISTOGRAM_BOOLEAN("TaskRunner.DispatchedTaskIsCancelled",
+                        pending_task.task.IsCancelled());
+
   ScopedTaskRunActivity task_activity(pending_task);
 
   tracked_objects::TaskStopwatch stopwatch;
   stopwatch.Start();
   tracked_objects::Duration queue_duration =
       stopwatch.StartTime() - pending_task.EffectiveTimePosted();
 
   TRACE_EVENT_WITH_FLOW1(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
                           queue_function,
                           TRACE_ID_MANGLE(GetTaskTraceID(pending_task)),
@@ skipping 17 matching lines @@
 }
 
 uint64_t TaskAnnotator::GetTaskTraceID(const PendingTask& task) const {
   return (static_cast<uint64_t>(task.sequence_num) << 32) |
          ((static_cast<uint64_t>(reinterpret_cast<intptr_t>(this)) << 32) >>
           32);
 }
 
 }  // namespace debug
 }  // namespace base