Measure how many tasks are cancelled in MessageLoop.

base/debug/task_annotator.cc

Index: base/debug/task_annotator.cc
diff --git a/base/debug/task_annotator.cc b/base/debug/task_annotator.cc
index 2747d63c2bf29aa3d356c8c750ff3784b943e66f..1f18dd03c618cac5ad03e3af20109f023f7f1c66 100644
--- a/base/debug/task_annotator.cc
+++ b/base/debug/task_annotator.cc
@@ -6,6 +6,7 @@
 
 #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"
@@ -29,6 +30,9 @@ void TaskAnnotator::DidQueueTask(const char* queue_function,
 
 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;