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

base/task_scheduler/task_traits.cc

Index: base/task_scheduler/task_traits.cc
diff --git a/base/task_scheduler/task_traits.cc b/base/task_scheduler/task_traits.cc
index dd555358522354b15f6d3acb57bf71db92aced7c..5bd868a330e1e5a75fa2d618d2474bc540211d4d 100644
--- a/base/task_scheduler/task_traits.cc
+++ b/base/task_scheduler/task_traits.cc
@@ -8,6 +8,8 @@
 
 #include <ostream>
 
+#include "base/logging.h"
+
 namespace base {
 
 // Do not rely on defaults hard-coded below beyond the guarantees described in
@@ -36,34 +38,60 @@ TaskTraits& TaskTraits::WithShutdownBehavior(
   return *this;
 }
 
-std::ostream& operator<<(std::ostream& os, const TaskPriority& task_priority) {
+const char* TaskPriorityToString(TaskPriority task_priority) {
   switch (task_priority) {
     case TaskPriority::BACKGROUND:
-      os << "BACKGROUND";
-      break;
+      return "BACKGROUND";
     case TaskPriority::USER_VISIBLE:
-      os << "USER_VISIBLE";
-      break;
+      return "USER_VISIBLE";
     case TaskPriority::USER_BLOCKING:
-      os << "USER_BLOCKING";
-      break;
+      return "USER_BLOCKING";
   }
-  return os;
+  NOTREACHED();
+  return "";
 }
 
-std::ostream& operator<<(std::ostream& os,
-                         const TaskShutdownBehavior& shutdown_behavior) {
+const char* TaskShutdownBehaviorToString(
+    TaskShutdownBehavior shutdown_behavior) {
   switch (shutdown_behavior) {
     case TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN:
-      os << "CONTINUE_ON_SHUTDOWN";
-      break;
+      return "CONTINUE_ON_SHUTDOWN";
     case TaskShutdownBehavior::SKIP_ON_SHUTDOWN:
-      os << "SKIP_ON_SHUTDOWN";
-      break;
+      return "SKIP_ON_SHUTDOWN";
     case TaskShutdownBehavior::BLOCK_SHUTDOWN:
-      os << "BLOCK_SHUTDOWN";
-      break;
+      return "BLOCK_SHUTDOWN";
   }
+  NOTREACHED();
+  return "";
+}
+
+const char* ExecutionModeToString(ExecutionMode execution_mode) {
+  switch (execution_mode) {
+    case ExecutionMode::PARALLEL:
+      return "PARALLEL";
+    case ExecutionMode::SEQUENCED:
+      return "SEQUENCED";
+    case ExecutionMode::SINGLE_THREADED:
+      return "SINGLE_THREADED";
+  }
+  NOTREACHED();
+  return "";
+}
+
+std::ostream& operator<<(std::ostream& os, const TaskPriority& task_priority) {
+  os << TaskPriorityToString(task_priority);
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         const TaskShutdownBehavior& shutdown_behavior) {
+  os << TaskShutdownBehaviorToString(shutdown_behavior);
+  return os;
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         const ExecutionMode& execution_mode) {
+  os << ExecutionModeToString(execution_mode);
   return os;
 }