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

base/test/scoped_task_environment.h

Index: base/test/scoped_task_environment.h
diff --git a/base/test/scoped_task_environment.h b/base/test/scoped_task_environment.h
index 2887d0765bacf5753eb4037a8f79931e22e9c022..b0a6db328f7227555a5688ad8d5b85e83136c1a9 100644
--- a/base/test/scoped_task_environment.h
+++ b/base/test/scoped_task_environment.h
@@ -32,7 +32,17 @@ namespace test {
 // https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit
 class ScopedTaskEnvironment {
  public:
-  ScopedTaskEnvironment();
+  enum class MainThreadType {
+    // The main thread doesn't pump messages.

[gab, 2017/04/27 14:22:59]

s/doesn't pump messages/doesn't pump system messages/ ?

+    DEFAULT,
+    // The main thread pumps UI messages.
+    UI,
+    // The main thread pumps asynchronous IO messages.
+    IO,
+  };
+
+  ScopedTaskEnvironment(
+      MainThreadType main_thread_type = MainThreadType::DEFAULT);
 
   // Runs pending (Thread|Sequenced)TaskRunnerHandle tasks and pending
   // BLOCK_SHUTDOWN TaskScheduler tasks. Then, unregisters the TaskScheduler and