First version of PerformanceObserver

Source/core/timing/PerformanceUserTiming.h

Index: Source/core/timing/PerformanceUserTiming.h
diff --git a/Source/core/timing/PerformanceUserTiming.h b/Source/core/timing/PerformanceUserTiming.h
index af68d36b5620f423409e86609544aae7f6693ebc..fbc4814061195def7cc859d6b490a472841bc77d 100644
--- a/Source/core/timing/PerformanceUserTiming.h
+++ b/Source/core/timing/PerformanceUserTiming.h
@@ -41,15 +41,15 @@ using PerformanceEntryMap = HeapHashMap<String, PerformanceEntryVector>;
 
 class UserTiming final : public GarbageCollected<UserTiming> {
 public:
-    static UserTiming* create(PerformanceBase* performance)
+    static UserTiming* create(PerformanceBase& performance)
     {
         return new UserTiming(performance);
     }
 
-    void mark(const String& markName, ExceptionState&);
+    PerformanceEntry* mark(const String& markName, ExceptionState&);
     void clearMarks(const String& markName);
 
-    void measure(const String& measureName, const String& startMark, const String& endMark, ExceptionState&);
+    PerformanceEntry* measure(const String& measureName, const String& startMark, const String& endMark, ExceptionState&);
     void clearMeasures(const String& measureName);
 
     PerformanceEntryVector getMarks() const;
@@ -61,7 +61,7 @@ public:
     DECLARE_TRACE();
 
 private:
-    explicit UserTiming(PerformanceBase*);
+    explicit UserTiming(PerformanceBase&);
 
     double findExistingMarkStartTime(const String& markName, ExceptionState&);