Make scoped enums output streamable.

base/task_scheduler/task_traits.h

Index: base/task_scheduler/task_traits.h
diff --git a/base/task_scheduler/task_traits.h b/base/task_scheduler/task_traits.h
index 523fd137b66557e7146f1774b661b8b8e3cf6d5e..efb8361cb8718d43eb67237ddea331ec21581cb5 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -126,15 +126,6 @@ enum class ExecutionMode {
   SINGLE_THREADED,
 };
 
-// Stream operators so TaskPriority and TaskShutdownBehavior can be used in
-// DCHECK statements.
-BASE_EXPORT std::ostream& operator<<(std::ostream& os,
-                                     const TaskPriority& shutdown_behavior);
-
-BASE_EXPORT std::ostream& operator<<(
-    std::ostream& os,
-    const TaskShutdownBehavior& shutdown_behavior);
-
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_