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..671d2066be9509e87b7cdd94418b71f7ecef6516 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -357,7 +357,7 @@ std::string MessageLoop::GetThreadName() const {
 void MessageLoop::SetTaskRunner(
     scoped_refptr<SingleThreadTaskRunner> task_runner) {
   DCHECK_EQ(this, current());
-  DCHECK(task_runner->BelongsToCurrentThread());
+  DCHECK(!task_runner || task_runner->BelongsToCurrentThread());
   DCHECK(!unbound_task_runner_);
   task_runner_ = std::move(task_runner);
   SetThreadTaskRunnerHandle();
@@ -368,7 +368,8 @@ void MessageLoop::SetThreadTaskRunnerHandle() {
   // Clear the previous thread task runner first, because only one can exist at
   // a time.
   thread_task_runner_handle_.reset();
-  thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner_));
+  if (task_runner_)
+    thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner_));
 }
 
 void MessageLoop::RunHandler() {