Convert SVGSMILElement::m_interval{Begin,End} into a SMILInterval

Source/core/svg/animation/SVGSMILElement.h

Index: Source/core/svg/animation/SVGSMILElement.h
diff --git a/Source/core/svg/animation/SVGSMILElement.h b/Source/core/svg/animation/SVGSMILElement.h
index f995d1950e7a0cdc49a78ebb8a7714cbf131065f..5404dd13aec8fbf31a6fe9d2b0dc2989d96bd8ac 100644
--- a/Source/core/svg/animation/SVGSMILElement.h
+++ b/Source/core/svg/animation/SVGSMILElement.h
@@ -87,7 +87,7 @@ public:
 
     SMILTime elapsed() const;
 
-    SMILTime intervalBegin() const { return m_intervalBegin; }
+    SMILTime intervalBegin() const { return m_interval.begin; }
     SMILTime previousIntervalBegin() const { return m_previousIntervalBegin; }
     SMILTime simpleDuration() const;
 
@@ -244,8 +244,7 @@ private:
     Vector<SMILTimeWithOrigin> m_endTimes;
 
     // This is the upcoming or current interval
-    SMILTime m_intervalBegin;
-    SMILTime m_intervalEnd;
+    SMILInterval m_interval;
 
     SMILTime m_previousIntervalBegin;