Run ScopedTaskScheduler tasks synchronously.

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 1581f6cfb4b494b543a4574c2ca786f18f631188..2212941db9ae15a3a6a30a590389dca826134f35 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -357,12 +357,20 @@ std::string MessageLoop::GetThreadName() const {
 void MessageLoop::SetTaskRunner(
     scoped_refptr<SingleThreadTaskRunner> task_runner) {
   DCHECK_EQ(this, current());
+  DCHECK(task_runner);
   DCHECK(task_runner->BelongsToCurrentThread());
   DCHECK(!unbound_task_runner_);
   task_runner_ = std::move(task_runner);
   SetThreadTaskRunnerHandle();
 }
 
+void MessageLoop::ClearTaskRunnerForTesting() {
+  DCHECK_EQ(this, current());
+  DCHECK(!unbound_task_runner_);
+  task_runner_ = nullptr;
+  thread_task_runner_handle_.reset();
+}
+
 void MessageLoop::SetThreadTaskRunnerHandle() {
   DCHECK_EQ(this, current());
   // Clear the previous thread task runner first, because only one can exist at