Fix comment in scoped_task_environment.h.

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 b0a6db328f7227555a5688ad8d5b85e83136c1a9..7f9387c02a06255e27fd83a7ca4bf4d553163d48 100644
--- a/base/test/scoped_task_environment.h
+++ b/base/test/scoped_task_environment.h
@@ -33,7 +33,7 @@ namespace test {
 class ScopedTaskEnvironment {
  public:
   enum class MainThreadType {
-    // The main thread doesn't pump messages.
+    // The main thread doesn't pump system messages.
     DEFAULT,
     // The main thread pumps UI messages.
     UI,