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

third_party/WebKit/public/platform/WebThread.h

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 #ifndef WebThread_h
 #define WebThread_h
 
 #include "WebCommon.h"
+
 #include <stdint.h>
 
 namespace blink {
+namespace scheduler {
+class TaskTimeObserver;
+}
+
 class WebScheduler;
 class WebTaskRunner;
 
 // Always an integer value.
 typedef uintptr_t PlatformThreadId;
 
 // Provides an interface to an embedder-defined thread implementation.
 //
 // Deleting the thread blocks until all pending, non-delayed tasks have been
 // run.
@@ skipping 14 matching lines @@
         virtual void didProcessTask() = 0;
     };
 
 
     // Returns a WebTaskRunner bound to the underlying scheduler's default task queue.
     virtual WebTaskRunner* getWebTaskRunner() { return nullptr; }
 
     virtual bool isCurrentThread() const = 0;
     virtual PlatformThreadId threadId() const { return 0; }
 
+    // TaskObserver is an object that receives task notifications from the MessageLoop
+    // NOTE: TaskObserver implementation should be extremely fast!
+    // This API is performance sensitive. Use only if you have a compelling reason.
     virtual void addTaskObserver(TaskObserver*) { }
     virtual void removeTaskObserver(TaskObserver*) { }
 
+    // TaskTimeObserver is an object that receives notifications for

[pfeldman, 2016/08/29 17:23:08]

Should we instead set the observer to prevent the abuse?

[panicker, 2016/08/29 17:46:23]

Sorry, could you elaborate on what you mean by "set the observer" ?

(To provide context: this API is to enable usage from Inspector Agent etc.)

[pfeldman, 2016/08/29 17:52:54]

You are saying here that I need to have a compelling reason to add another
observer. But since we only have single client at this point, should we not
allow adding observers at all and introduce SetTaskTimeObserver for now? That
would eliminate potential abuse.

[panicker, 2016/08/29 18:01:32]

Reflector will also need this imminently as MessageLoop tasks is in their first
milestone, so it's not just single client.

[pfeldman, 2016/08/29 18:04:27]

Ok, I must say I am not a fan of it (I'd prefer single entity to collect the
data and others to sample it), but let us land this to unblock you and see how
reflector jumps on board and reuses it. We will then revisit it if reflector and
webperf gain a lot of similarity.

lgtm.

+    // CPU time spent in each top-level MessageLoop task.
+    // NOTE: TaskTimeObserver implementation should be extremely fast!
+    // This API is performance sensitive. Use only if you have a compelling reason.
+    virtual void addTaskTimeObserver(scheduler::TaskTimeObserver*) {}
+    virtual void removeTaskTimeObserver(scheduler::TaskTimeObserver*) {}
+
     // Returns the scheduler associated with the thread.
     virtual WebScheduler* scheduler() const = 0;
 
     virtual ~WebThread() { }
 };
 
 } // namespace blink
 
 #endif