Separate the create and start phases in TaskSchedulerImpl.

base/test/scoped_task_scheduler.cc

Index: base/test/scoped_task_scheduler.cc
diff --git a/base/test/scoped_task_scheduler.cc b/base/test/scoped_task_scheduler.cc
index 3c5b0e2fc7807a6b14ecddd640109a46c26aa417..ba19f628f3f882f1bfb74850c4d6a39c585ccc1c 100644
--- a/base/test/scoped_task_scheduler.cc
+++ b/base/test/scoped_task_scheduler.cc
@@ -38,6 +38,9 @@ namespace {
 
 enum class ExecutionMode { PARALLEL, SEQUENCED, SINGLE_THREADED };
 
+// ScopedTaskScheduler intentionally breaks the TaskScheduler contract of not
+// running tasks before Start(). This avoid having to call Start() with dummy
+// parameters.
 class TestTaskScheduler : public TaskScheduler {
  public:
   // |external_message_loop| is an externally provided MessageLoop on which to
@@ -47,6 +50,7 @@ class TestTaskScheduler : public TaskScheduler {
   ~TestTaskScheduler() override;
 
   // TaskScheduler:
+  void Start(const TaskScheduler::InitParams& init_params) override;
   void PostDelayedTaskWithTraits(const tracked_objects::Location& from_here,
                                  const TaskTraits& traits,
                                  OnceClosure task,
@@ -164,6 +168,10 @@ TestTaskScheduler::~TestTaskScheduler() {
     Shutdown();
 }
 
+void TestTaskScheduler::Start(const TaskScheduler::InitParams&) {
+  NOTREACHED();
+}
+
 void TestTaskScheduler::PostDelayedTaskWithTraits(
     const tracked_objects::Location& from_here,
     const TaskTraits& traits,