Add foundation for removing AnimatableValues from StyleInterpolation

Source/core/animation/Interpolation.cpp

 // 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.
 
 #include "config.h"
 #include "core/animation/Interpolation.h"
 
 namespace blink {
 
 namespace {
 
 bool typesMatch(const InterpolableValue* start, const InterpolableValue* end)
 {
+    if (start == end)
+        return true;
     if (start->isNumber())
         return end->isNumber();
     if (start->isBool())
         return end->isBool();
     if (start->isAnimatableValue())
         return end->isAnimatableValue();
     if (!(start->isList() && end->isList()))
         return false;
     const InterpolableList* startList = toInterpolableList(start);
     const InterpolableList* endList = toInterpolableList(end);
     if (startList->length() != endList->length())
         return false;
     for (size_t i = 0; i < startList->length(); ++i) {
         if (!typesMatch(startList->get(i), endList->get(i)))
             return false;
     }
     return true;
 }
 
 }
 
 Interpolation::Interpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end)
     : m_start(start)
     , m_end(end)
     , m_cachedFraction(0)
     , m_cachedIteration(0)
-    , m_cachedValue(m_start->clone())
+    , m_cachedValue(m_start ? m_start->clone() : nullptr)
 {
     RELEASE_ASSERT(typesMatch(m_start.get(), m_end.get()));
 }
 
 Interpolation::~Interpolation()
 {
 }
 
-void Interpolation::interpolate(int iteration, double fraction) const
+void Interpolation::interpolate(int iteration, double fraction)
 {
     if (m_cachedFraction != fraction || m_cachedIteration != iteration) {
         m_start->interpolate(*m_end, fraction, *m_cachedValue);
         m_cachedIteration = iteration;
         m_cachedFraction = fraction;
     }
 }
 
 DEFINE_TRACE(Interpolation)
 {
     visitor->trace(m_start);
     visitor->trace(m_end);
     visitor->trace(m_cachedValue);
 }
 
 }