Allow ScopedTaskEnvironment to pump UI or IO messages on the main thread.

base/test/scoped_task_environment.cc

Index: base/test/scoped_task_environment.cc
diff --git a/base/test/scoped_task_environment.cc b/base/test/scoped_task_environment.cc
index b18bf6a19d3a940ba0268f1dcb4409780d108cf5..9f4aed5c0c0f9b916157772d68344e91f12e143b 100644
--- a/base/test/scoped_task_environment.cc
+++ b/base/test/scoped_task_environment.cc
@@ -12,7 +12,10 @@
 namespace base {
 namespace test {
 
-ScopedTaskEnvironment::ScopedTaskEnvironment() {
+ScopedTaskEnvironment::ScopedTaskEnvironment(MainThreadType main_thread_type)
+    : message_loop_(main_thread_type == MainThreadType::UI
+                        ? MessageLoop::TYPE_UI
+                        : MessageLoop::TYPE_IO) {
   DCHECK(!TaskScheduler::GetInstance());
 
   // Instantiate a TaskScheduler with 1 thread in each of its 4 pools. Threads