Run ScopedTaskScheduler tasks synchronously.

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index ac7a30356366ed0d37f32f0498f3822cfb714896..91a7b1d33268d470ee4220202dd45734bc3d01ec 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -234,6 +234,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // thread to which the message loop is bound.
   void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
 
+  // Clears task_runner() and the ThreadTaskRunnerHandle for the target thread.
+  // Must be called on the thread to which the message loop is bound.
+  void ClearTaskRunnerForTesting();
+
   // Enables or disables the recursive task processing. This happens in the case
   // of recursive message loops. Some unwanted message loops may occur when
   // using common controls or printer functions. By default, recursive task