Add additive animation support for CSS property background-position

third_party/WebKit/Source/core/animation/InterpolableValue.h

 // Copyright 2014 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 InterpolableValue_h
 #define InterpolableValue_h
 
 #include "core/CoreExport.h"
 #include "core/animation/animatable/AnimatableValue.h"
 #include "platform/heap/Handle.h"
@@ skipping 43 matching lines @@
         return adoptPtr(new InterpolableNumber(value));
     }
 
     bool isNumber() const final { return true; }
     double value() const { return m_value; }
     bool equals(const InterpolableValue& other) const final;
     PassOwnPtr<InterpolableValue> clone() const final { return create(m_value); }
     PassOwnPtr<InterpolableValue> cloneAndZero() const final { return create(0); }
     void scale(double scale) final;
     void scaleAndAdd(double scale, const InterpolableValue& other) final;
+    void set(double value) { m_value = value; }
 
 private:
     void interpolate(const InterpolableValue& to, const double progress, InterpolableValue& result) const final;
     double m_value;
 
     explicit InterpolableNumber(double value)
         : m_value(value)
     {
     }
 
@@ skipping 115 matching lines @@
 };
 
 DEFINE_TYPE_CASTS(InterpolableNumber, InterpolableValue, value, value->isNumber(), value.isNumber());
 DEFINE_TYPE_CASTS(InterpolableBool, InterpolableValue, value, value->isBool(), value.isBool());
 DEFINE_TYPE_CASTS(InterpolableList, InterpolableValue, value, value->isList(), value.isList());
 DEFINE_TYPE_CASTS(InterpolableAnimatableValue, InterpolableValue, value, value->isAnimatableValue(), value.isAnimatableValue());
 
 } // namespace blink
 
 #endif