enable / disable Long Task instrumentation based on presence of LongTask observer

third_party/WebKit/Source/core/timing/PerformanceBase.h

Index: third_party/WebKit/Source/core/timing/PerformanceBase.h
diff --git a/third_party/WebKit/Source/core/timing/PerformanceBase.h b/third_party/WebKit/Source/core/timing/PerformanceBase.h
index 463faf12043d5c14d5a53361cd21ad5c543977d0..10b2f4f324fc9724d37ee2719313368927da7550 100644
--- a/third_party/WebKit/Source/core/timing/PerformanceBase.h
+++ b/third_party/WebKit/Source/core/timing/PerformanceBase.h
@@ -63,6 +63,8 @@ public:
 
     virtual PerformanceTiming* timing() const;
 
+    virtual void updateLongTaskInstrumentation() {}
+
     // Reduce the resolution to 5µs to prevent timing attacks. See:
     // http://www.w3.org/TR/hr-time-2/#privacy-security
     static double clampTimeResolution(double timeSeconds);
@@ -132,7 +134,7 @@ protected:
     void addLongTaskTimingBuffer(PerformanceEntry&);
 
     void notifyObserversOfEntry(PerformanceEntry&);
-    bool hasObserverFor(PerformanceEntry::EntryType);
+    bool hasObserverFor(PerformanceEntry::EntryType) const;
 
     void deliverObservationsTimerFired(TimerBase*);