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

base/task_scheduler/task_traits.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 #define BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 
 #include <stdint.h>
 
 #include <iosfwd>
@@ skipping 108 matching lines @@
   PARALLEL,
 
   // Executes one task at a time in posting order. The sequence’s priority is
   // equivalent to the highest priority pending task in the sequence.
   SEQUENCED,
 
   // Executes one task at a time on a single thread in posting order.
   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
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_