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

third_party/WebKit/public/platform/WebThread.h

Index: third_party/WebKit/public/platform/WebThread.h
diff --git a/third_party/WebKit/public/platform/WebThread.h b/third_party/WebKit/public/platform/WebThread.h
index c9a45b6f57ed9ab753eaf8769be1054f4526a3ff..1a054dcdd7d8beec7c0b5e27fd277c5caf3a4275 100644
--- a/third_party/WebKit/public/platform/WebThread.h
+++ b/third_party/WebKit/public/platform/WebThread.h
@@ -56,6 +56,13 @@ public:
         virtual void didProcessTask() = 0;
     };
 
+    class BLINK_PLATFORM_EXPORT TaskTimeObserver {
+    public:
+        virtual ~TaskTimeObserver() { }
+        virtual void willProcessTask() = 0;

[Sami, 2016/08/19 14:44:56]

Could you remind me why we need did/willProcess again? In any case they're
redundant with TaskObserver so should probably just reuse that.

[panicker, 2016/08/19 18:06:50]

This is to line up the Begin / End with Time Reporting exactly.
For instance, I think I will get additional willProcess / DidProcess without
corresponding reportTaskTime -- because of the !delegate check in
task_queue_manager (the condition is present for reportTaskTime but not for the
willProcess / DidProcess)

[panicker, 2016/08/19 18:10:53]

If you can suggest a completely different way to hook up WebPerfAgent to the
scheduler (i.e. not involving WebThread) that might work too. Thoughts?

[Sami, 2016/08/22 15:51:30]

The task observer can also check if it is nested. Basically you'll get another
willProcess() call before the corresponding didProcess().

+        virtual void didProcessTask() = 0;
+        virtual void reportTaskTime(double, double) = 0;
+    };
 
     // Returns a WebTaskRunner bound to the underlying scheduler's default task queue.
     virtual WebTaskRunner* getWebTaskRunner() { return nullptr; }
@@ -65,6 +72,8 @@ public:
 
     virtual void addTaskObserver(TaskObserver*) { }
     virtual void removeTaskObserver(TaskObserver*) { }
+    virtual void addTaskTimeObserver(TaskTimeObserver*) { }
+    virtual void removeTaskTimeObserver(TaskTimeObserver*) { }
 
     // Returns the scheduler associated with the thread.
     virtual WebScheduler* scheduler() const = 0;