Fix smooth scroll animation flake on janky pages

cc/test/animation_test_common.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TEST_ANIMATION_TEST_COMMON_H_
 #define CC_TEST_ANIMATION_TEST_COMMON_H_
 
 #include "cc/animation/animation.h"
 #include "cc/animation/animation_curve.h"
 #include "cc/animation/animation_timeline.h"
 #include "cc/animation/element_id.h"
 #include "cc/animation/transform_operations.h"
 #include "cc/output/filter_operations.h"
 #include "cc/test/geometry_test_utils.h"
 
 namespace cc {
 class AnimationPlayer;
 class ElementAnimations;
 class LayerImpl;
 class Layer;
 }
 
+namespace gfx {
+class ScrollOffset;
+}
+
 namespace cc {
 
 class FakeFloatAnimationCurve : public FloatAnimationCurve {
  public:
   FakeFloatAnimationCurve();
   explicit FakeFloatAnimationCurve(double duration);
   ~FakeFloatAnimationCurve() override;
 
   base::TimeDelta Duration() const override;
   float GetValue(base::TimeDelta now) const override;
@@ skipping 35 matching lines @@
   float GetValue(base::TimeDelta time) const override;
 
   std::unique_ptr<AnimationCurve> Clone() const override;
 
  private:
   base::TimeDelta duration_;
   float from_;
   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,
                                             float end_opacity,
                                             bool use_timing_function);
 
 int AddAnimatedTransformToElementAnimations(ElementAnimations* target,
                                             double duration,
                                             int delta_x,
                                             int delta_y);
@@ skipping 78 matching lines @@
     bool use_timing_function);
 
 void AbortAnimationsOnElementWithPlayer(
     ElementId element_id,
     scoped_refptr<AnimationTimeline> timeline,
     TargetProperty::Type target_property);
 
 }  // namespace cc
 
 #endif  // CC_TEST_ANIMATION_TEST_COMMON_H_