Add TaskScheduler::JoinForTesting().

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 a57802c300ec35d24db315ceb9a273e4c8484aaa..f411a0dbea24de9a75786d24a18bd4e3fffd8c5e 100644
--- a/base/test/scoped_task_scheduler.cc
+++ b/base/test/scoped_task_scheduler.cc
@@ -53,6 +53,7 @@ class TestTaskScheduler : public TaskScheduler {
   std::vector<const HistogramBase*> GetHistograms() const override;
   void Shutdown() override;
   void FlushForTesting() override;
+  void JoinForTesting() override;
 
   // Posts |task| to this TaskScheduler with |sequence_token|. Returns true on
   // success.
@@ -168,6 +169,10 @@ void TestTaskScheduler::FlushForTesting() {
   NOTREACHED();
 }
 
+void TestTaskScheduler::JoinForTesting() {
+  NOTREACHED();
+}
+
 bool TestTaskScheduler::PostTask(std::unique_ptr<internal::Task> task,
                                  const SequenceToken& sequence_token) {
   DCHECK(task);