Web Animations: Add a fast path for LengthStyleIntepolation application

Source/core/animation/LengthStyleInterpolation.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 LengthStyleInterpolation_h
 #define LengthStyleInterpolation_h
 
 #include "core/animation/StyleInterpolation.h"
 #include "core/css/CSSPrimitiveValue.h"
-#include "platform/Length.h"
 
 namespace blink {
 
+class LayoutStyle;
+class Length;
+
 class LengthStyleInterpolation : public StyleInterpolation {
 public:
-
+    typedef void (LayoutStyle::*LengthSetter)(const Length&);
     typedef void NonInterpolableType;
 
     static PassRefPtrWillBeRawPtr<LengthStyleInterpolation> create(const CSSValue& start, const CSSValue& end, CSSPropertyID id, InterpolationRange range)
     {
         return adoptRefWillBeNoop(new LengthStyleInterpolation(toInterpolableValue(start), toInterpolableValue(end), id, range));
     }
 
     static bool canCreateFrom(const CSSValue&);
 
     virtual void apply(StyleResolverState&) const override;
 
     DECLARE_VIRTUAL_TRACE();
 
     static PassOwnPtrWillBeRawPtr<InterpolableValue> toInterpolableValue(const CSSValue&);
     static PassRefPtrWillBeRawPtr<CSSPrimitiveValue> fromInterpolableValue(const InterpolableValue&, InterpolationRange);
 
 private:
-    LengthStyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id,  InterpolationRange range)
+    LengthStyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id, InterpolationRange range)
         : StyleInterpolation(start, end, id)
         , m_range(range)
-        { }
+        , m_lengthSetter(nullptr)
+    {
+        if (isPixelsOrPercentOnly(*m_start) && isPixelsOrPercentOnly(*m_end))
+            m_lengthSetter = lengthSetterForProperty(id);
+    }
+
+    static bool isPixelsOrPercentOnly(const InterpolableValue&);
+    static LengthSetter lengthSetterForProperty(CSSPropertyID);
 
     InterpolationRange m_range;
+    LengthSetter m_lengthSetter;
 };
 
 }
 
 #endif