Add smooth interpolation support for <length> custom properties

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

Index: third_party/WebKit/Source/core/animation/LengthInterpolationFunctions.h
diff --git a/third_party/WebKit/Source/core/animation/LengthInterpolationFunctions.h b/third_party/WebKit/Source/core/animation/LengthInterpolationFunctions.h
index 58006fcdd395ff569bf3335ba98303469b3bb0e0..04112fdbb1bf12da267ba1a6ac8f38de8a508ace 100644
--- a/third_party/WebKit/Source/core/animation/LengthInterpolationFunctions.h
+++ b/third_party/WebKit/Source/core/animation/LengthInterpolationFunctions.h
@@ -39,6 +39,12 @@ class LengthInterpolationFunctions {
                              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);
 };