Move and rename TaskTimeTracker to public interface exposed to WebThread, use in WebPerf Agent.

third_party/WebKit/public/platform/scheduler/child/webthread_base.h

Index: third_party/WebKit/public/platform/scheduler/child/webthread_base.h
diff --git a/third_party/WebKit/public/platform/scheduler/child/webthread_base.h b/third_party/WebKit/public/platform/scheduler/child/webthread_base.h
index 6bfddec80336f03badd23dc4c801e033027a304c..d5f059d5c4050bc9ea0e97e9b227f2120275b39a 100644
--- a/third_party/WebKit/public/platform/scheduler/child/webthread_base.h
+++ b/third_party/WebKit/public/platform/scheduler/child/webthread_base.h
@@ -33,6 +33,9 @@ class BLINK_PLATFORM_EXPORT WebThreadBase : public WebThread {
   void addTaskObserver(TaskObserver* observer) override;
   void removeTaskObserver(TaskObserver* observer) override;
 
+  void addTaskTimeObserver(TaskTimeObserver* task_time_observer) override;
+  void removeTaskTimeObserver(TaskTimeObserver* task_time_observer) override;
+
   // Returns the base::Bind-compatible task runner for posting tasks to this
   // thread. Can be called from any thread.
   virtual base::SingleThreadTaskRunner* GetTaskRunner() const = 0;
@@ -51,6 +54,9 @@ class BLINK_PLATFORM_EXPORT WebThreadBase : public WebThread {
   virtual void RemoveTaskObserverInternal(
       base::MessageLoop::TaskObserver* observer);
 
+  virtual void AddTaskTimeObserverInternal(TaskTimeObserver*) {}
+  virtual void RemoveTaskTimeObserverInternal(TaskTimeObserver*) {}
+
   static void RunWebThreadIdleTask(
       std::unique_ptr<WebThread::IdleTask> idle_task,
       base::TimeTicks deadline);