First version of PerformanceObserver

Source/core/timing/PerformanceResourceTiming.h

Index: Source/core/timing/PerformanceResourceTiming.h
diff --git a/Source/core/timing/PerformanceResourceTiming.h b/Source/core/timing/PerformanceResourceTiming.h
index 3a23c2ecc5b1b422366a156f79679961ba4f1cdf..0ec1909d4344de894f59e1b3ef540fa74f139bc3 100644
--- a/Source/core/timing/PerformanceResourceTiming.h
+++ b/Source/core/timing/PerformanceResourceTiming.h
@@ -69,8 +69,6 @@ public:
     double responseStart() const;
     double responseEnd() const;
 
-    bool isResource() override { return true; }
-
 private:
     PerformanceResourceTiming(const ResourceTimingInfo&, double timeOrigin, double startTime, double lastRedirectEndTime, bool m_allowTimingDetails, bool m_allowRedirectDetails);
     ~PerformanceResourceTiming() override;