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());

[dcheng, 2016/12/15 06:47:36]

I would prefer we limit the API changes here to be available in tests only
(though I'm not sure if that's possible). Or are there other situations where
this would be useful?

[fdoray, 2016/12/15 16:39:31]

Done.

   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() {