Add smooth interpolation support for <length> custom properties

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

 // Copyright 2016 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 LengthInterpolationFunctions_h
 #define LengthInterpolationFunctions_h
 
 #include "core/animation/InterpolationValue.h"
 #include "core/animation/PairwiseInterpolationValue.h"
 #include "platform/Length.h"
@@ skipping 21 matching lines @@
   static void composite(std::unique_ptr<InterpolableValue>&,
                         RefPtr<NonInterpolableValue>&,
                         double underlyingFraction,
                         const InterpolableValue&,
                         const NonInterpolableValue*);
   static Length createLength(const InterpolableValue&,
                              const NonInterpolableValue*,
                              const CSSToLengthConversionData&,
                              ValueRange);
 
+  // Unlike createLength() this preserves all specificed unit types via calc()
+  // expressions.
+  static const CSSValue* createCSSValue(const InterpolableValue&,
+                                        const NonInterpolableValue*,
+                                        ValueRange);
+
   static void subtractFromOneHundredPercent(InterpolationValue& result);
 };
 
 }  // namespace blink
 
 #endif  // LengthInterpolationFunctions_h