Implement CSSOM smooth scroll for Elements

Source/platform/scroll/ProgrammaticScrollAnimator.cpp

Index: Source/platform/scroll/ProgrammaticScrollAnimator.cpp
diff --git a/Source/platform/scroll/ProgrammaticScrollAnimator.cpp b/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
index e7478adb5354ce82d92283c733f71a0793103d19..574018064f1a97aeb528ca6e7a317ae5dec6e2aa 100644
--- a/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
+++ b/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
@@ -41,6 +41,7 @@ void ProgrammaticScrollAnimator::animateToOffset(FloatPoint offset)
     m_animationCurve = adoptPtr(Platform::current()->compositorSupport()->createScrollOffsetAnimationCurve(m_targetOffset, WebCompositorAnimationCurve::TimingFunctionTypeEaseInOut));
 
     m_animationCurve->setInitialValue(FloatPoint(m_scrollableArea->scrollPosition()));
+    m_scrollableArea->registerForAnimation();
     if (!m_scrollableArea->scheduleAnimation()) {
         resetAnimationState();
         m_scrollableArea->notifyScrollPositionChanged(IntPoint(offset.x(), offset.y()));
@@ -71,4 +72,9 @@ void ProgrammaticScrollAnimator::tickAnimation(double monotonicTime)
     }
 }
 
+bool ProgrammaticScrollAnimator::hasRunningAnimation() const
+{
+    return !!m_animationCurve;
+}
+
 } // namespace blink