Fix smooth scroll animation flake on janky pages

cc/test/animation_test_common.h

Index: cc/test/animation_test_common.h
diff --git a/cc/test/animation_test_common.h b/cc/test/animation_test_common.h
index 5c9e6b2e262065852b445d4c79f471cd59ea264c..e0bfe8645c6a95d4237efa88b35c0577bc3d7b4b 100644
--- a/cc/test/animation_test_common.h
+++ b/cc/test/animation_test_common.h
@@ -20,6 +20,10 @@ class LayerImpl;
 class Layer;
 }
 
+namespace gfx {
+class ScrollOffset;
+}
+
 namespace cc {
 
 class FakeFloatAnimationCurve : public FloatAnimationCurve {
@@ -75,6 +79,11 @@ class FakeFloatTransition : public FloatAnimationCurve {
   float to_;
 };
 
+int AddScrollOffsetAnimationToElementAnimations(ElementAnimations* target,
+                                                gfx::ScrollOffset initial_value,
+                                                gfx::ScrollOffset target_value,
+                                                bool impl_only);
+
 int AddOpacityTransitionToElementAnimations(ElementAnimations* target,
                                             double duration,
                                             float start_opacity,