Add a task_scheduler tracing category which will record an extra event per task.

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 0c0d304dcf53edf41f679904e3b45dc296a1b911..9fee22427bef130103475466166db6d976955c52 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -126,14 +126,22 @@ enum class ExecutionMode {
   SINGLE_THREADED,
 };
 
-// Stream operators so TaskPriority and TaskShutdownBehavior can be used in
-// DCHECK statements.
+// Returns string literals for the enums defined in this file. These methods
+// should only be used for tracing and debugging.
+BASE_EXPORT const char* TaskPriorityToString(TaskPriority task_priority);
+BASE_EXPORT const char* TaskShutdownBehaviorToString(
+    TaskShutdownBehavior task_priority);
+BASE_EXPORT const char* ExecutionModeToString(ExecutionMode task_priority);
+
+// Stream operators so that the enums defined in this file can be used in
+// DCHECK and EXPECT 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);
+BASE_EXPORT std::ostream& operator<<(std::ostream& os,
+                                     const ExecutionMode& execution_mode);
 
 }  // namespace base