Add ScopedTaskEnvironment::ExecutionControlMode.

base/task_scheduler/task_tracker.cc

Index: base/task_scheduler/task_tracker.cc
diff --git a/base/task_scheduler/task_tracker.cc b/base/task_scheduler/task_tracker.cc
index 55230aa93742ee3c58da9d43a2a67dc72d452cd8..89ea3e7c7f6e0d3bd14f685fd50202fe0ebe8162 100644
--- a/base/task_scheduler/task_tracker.cc
+++ b/base/task_scheduler/task_tracker.cc
@@ -388,6 +388,12 @@ bool TaskTracker::IsPostingBlockShutdownTaskAfterShutdownAllowed() {
 }
 #endif
 
+int TaskTracker::GetNumPendingUndelayedTasksForTesting() const {
+  // No barrier is required, but there is no function in base/ to read an atomic
+  // variable without a barrier.
+  return subtle::Acquire_Load(&num_pending_undelayed_tasks_);
+}
+
 bool TaskTracker::BeforePostTask(TaskShutdownBehavior shutdown_behavior) {
   if (shutdown_behavior == TaskShutdownBehavior::BLOCK_SHUTDOWN) {
     // BLOCK_SHUTDOWN tasks block shutdown between the moment they are posted