Add ScopedTaskEnvironment::ExecutionControlMode.

base/task_scheduler/task_scheduler_impl.h

Index: base/task_scheduler/task_scheduler_impl.h
diff --git a/base/task_scheduler/task_scheduler_impl.h b/base/task_scheduler/task_scheduler_impl.h
index 4907ce2df266e91e6aac6de9df4f213a1fa419dd..1e35639958b62aa6aced77ca1bb71fbb028f70dc 100644
--- a/base/task_scheduler/task_scheduler_impl.h
+++ b/base/task_scheduler/task_scheduler_impl.h
@@ -12,6 +12,7 @@
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/atomic_flag.h"
@@ -38,8 +39,19 @@ namespace internal {
 // Default TaskScheduler implementation. This class is thread-safe.
 class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
  public:
-  // |name| is used to label threads and histograms.
-  explicit TaskSchedulerImpl(StringPiece name);
+  using TaskTrackerImpl =
+#if defined(OS_POSIX) && !defined(OS_NACL_SFI)
+      TaskTrackerPosix;
+#else
+      TaskTracker;
+#endif
+
+  // |name| is used to label threads and histograms. |task_tracker| can be used
+  // for tests that need more execution control. By default, the production
+  // TaskTracker is used.
+  explicit TaskSchedulerImpl(StringPiece name,
+                             std::unique_ptr<TaskTrackerImpl> task_tracker =
+                                 MakeUnique<TaskTrackerImpl>());
   ~TaskSchedulerImpl() override;
 
   // TaskScheduler:
@@ -74,11 +86,7 @@ class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
 
   const std::string name_;
   Thread service_thread_;
-#if defined(OS_POSIX) && !defined(OS_NACL_SFI)
-  TaskTrackerPosix task_tracker_;
-#else
-  TaskTracker task_tracker_;
-#endif
+  const std::unique_ptr<TaskTrackerImpl> task_tracker_;
   DelayedTaskManager delayed_task_manager_;
   SchedulerSingleThreadTaskRunnerManager single_thread_task_runner_manager_;