Add TaskScheduler.NumTasksBetweenWaits.[pool] histogram.

base/task_scheduler/scheduler_worker_pool_impl.h

Index: base/task_scheduler/scheduler_worker_pool_impl.h
diff --git a/base/task_scheduler/scheduler_worker_pool_impl.h b/base/task_scheduler/scheduler_worker_pool_impl.h
index 3ab2299af97a1f37a0e40fc886a4404123b6e440..a621f797a53473b2096170a07689e456d5c3f2de 100644
--- a/base/task_scheduler/scheduler_worker_pool_impl.h
+++ b/base/task_scheduler/scheduler_worker_pool_impl.h
@@ -94,6 +94,10 @@ class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
                                scoped_refptr<Sequence> sequence,
                                SchedulerWorker* worker) override;
 
+  const HistogramBase* num_tasks_between_waits_histogram_for_testing() const {
+    return num_tasks_between_waits_histogram_;
+  }
+
  private:
   class SchedulerSingleThreadTaskRunner;
   class SchedulerWorkerDelegateImpl;
@@ -177,6 +181,10 @@ class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
   // leaked.
   HistogramBase* const detach_duration_histogram_;
 
+  // TaskScheduler.NumTasksBetweenWaits.[worker pool name] histogram.
+  // Intentionally leaked.
+  HistogramBase* const num_tasks_between_waits_histogram_;
+
   // TaskScheduler.TaskLatency.[worker pool name].[task priority] histograms.
   // Indexed by task priority. Histograms are allocated on demand to reduce
   // memory usage (some task priorities might never run in this