Use high precision timestamp for Event.timestamp

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index f557c6f4bd34882fe3a8acf3142abe916e95cf31..7a85ada616b116c41105ac0b2e69594d5b1f78ec 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -912,6 +912,8 @@ public:
 
     const DocumentTiming& timing() const { return m_documentTiming; }
 
+    double monotonicTimeToZeroBasedDocumentTime(double) const;
+
     int requestAnimationFrame(FrameRequestCallback*);
     void cancelAnimationFrame(int id);
     void serviceScriptedAnimations(double monotonicAnimationStartTime);