Reland of un smooth scroll animations on the compositor when possible

third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp

Index: third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp b/third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
index f4a0af17641114168a3acfc23552c684be800f1c..695718ddd33e28c9bddcf78fb6e97fb984b9d00d 100644
--- a/third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
@@ -63,6 +63,12 @@
         notifyPositionChanged(IntPoint(offset.x(), offset.y()));
     }
     m_runState = RunState::WaitingToSendToCompositor;
+}
+
+void ProgrammaticScrollAnimator::cancelAnimation()
+{
+    ASSERT(m_runState != RunState::RunningOnCompositorButNeedsUpdate);
+    ScrollAnimatorCompositorCoordinator::cancelAnimation();
 }
 
 void ProgrammaticScrollAnimator::tickAnimation(double monotonicTime)
@@ -155,6 +161,7 @@
 
 void ProgrammaticScrollAnimator::notifyCompositorAnimationFinished(int groupId)
 {
+    ASSERT(m_runState != RunState::RunningOnCompositorButNeedsUpdate);
     ScrollAnimatorCompositorCoordinator::compositorAnimationFinished(groupId);
 }