Implemented additive animations for length

Source/core/animation/KeyframeEffectModel.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 122 matching lines @@
 }
 
 void KeyframeEffectModelBase::ensureInterpolationEffect(Element* element) const
 {
     if (m_interpolationEffect)
         return;
     m_interpolationEffect = InterpolationEffect::create();
 
     for (const auto& entry : *m_keyframeGroups) {
         const PropertySpecificKeyframeVector& keyframes = entry.value->keyframes();
-        ASSERT(keyframes[0]->composite() == AnimationEffect::CompositeReplace);
         for (size_t i = 0; i < keyframes.size() - 1; i++) {
-            ASSERT(keyframes[i + 1]->composite() == AnimationEffect::CompositeReplace);
             double applyFrom = i ? keyframes[i]->offset() : (-std::numeric_limits<double>::infinity());
             double applyTo = i == keyframes.size() - 2 ? std::numeric_limits<double>::infinity() : keyframes[i + 1]->offset();
             if (applyTo == 1)
                 applyTo = std::numeric_limits<double>::infinity();
 
             m_interpolationEffect->addInterpolationsFromKeyframes(entry.key, element, *keyframes[i], *keyframes[i + 1], applyFrom, applyTo);
         }
     }
 }
 
@@ skipping 70 matching lines @@
 }
 
 void KeyframeEffectModelBase::PropertySpecificKeyframeGroup::trace(Visitor* visitor)
 {
 #if ENABLE(OILPAN)
     visitor->trace(m_keyframes);
 #endif
 }
 
 } // namespace