Add ScopedTaskEnvironment::ExecutionControlMode.

base/task_scheduler/task_scheduler_impl.cc

Index: base/task_scheduler/task_scheduler_impl.cc
diff --git a/base/task_scheduler/task_scheduler_impl.cc b/base/task_scheduler/task_scheduler_impl.cc
index b1b2d36c11696d280c43387a4f209c767f2bb706..34a944fd054c767207488c6b1e6d00dcf7b4d0ec 100644
--- a/base/task_scheduler/task_scheduler_impl.cc
+++ b/base/task_scheduler/task_scheduler_impl.cc
@@ -6,7 +6,6 @@
 
 #include <utility>
 
-#include "base/memory/ptr_util.h"
 #include "base/task_scheduler/delayed_task_manager.h"
 #include "base/task_scheduler/scheduler_worker_pool_params.h"
 #include "base/task_scheduler/sequence.h"
@@ -53,10 +52,13 @@ size_t GetEnvironmentIndexForTraits(const TaskTraits& traits) {
 
 }  // namespace
 
-TaskSchedulerImpl::TaskSchedulerImpl(StringPiece name)
+TaskSchedulerImpl::TaskSchedulerImpl(
+    StringPiece name,
+    std::unique_ptr<TaskTrackerType> task_tracker)
     : name_(name),
       service_thread_("TaskSchedulerServiceThread"),
-      single_thread_task_runner_manager_(&task_tracker_,
+      task_tracker_(std::move(task_tracker)),
+      single_thread_task_runner_manager_(task_tracker_.get(),
                                          &delayed_task_manager_) {
   static_assert(arraysize(worker_pools_) == ENVIRONMENT_COUNT,
                 "The size of |worker_pools_| must match ENVIRONMENT_COUNT.");
@@ -68,7 +70,7 @@ TaskSchedulerImpl::TaskSchedulerImpl(StringPiece name)
        ++environment_type) {
     worker_pools_[environment_type] = MakeUnique<SchedulerWorkerPoolImpl>(
         name_ + kEnvironmentParams[environment_type].name_suffix,
-        kEnvironmentParams[environment_type].priority_hint, &task_tracker_,
+        kEnvironmentParams[environment_type].priority_hint, task_tracker_.get(),
         &delayed_task_manager_);
   }
 }
@@ -179,11 +181,11 @@ int TaskSchedulerImpl::GetMaxConcurrentTasksWithTraitsDeprecated(
 
 void TaskSchedulerImpl::Shutdown() {
   // TODO(fdoray): Increase the priority of BACKGROUND tasks blocking shutdown.
-  task_tracker_.Shutdown();
+  task_tracker_->Shutdown();
 }
 
 void TaskSchedulerImpl::FlushForTesting() {
-  task_tracker_.Flush();
+  task_tracker_->Flush();
 }
 
 void TaskSchedulerImpl::JoinForTesting() {