[WIP] Web Animations API: Constructing an Animation from partial keyframes throws a JS exception

Source/core/animation/KeyframeEffectModel.cpp

Index: Source/core/animation/KeyframeEffectModel.cpp
diff --git a/Source/core/animation/KeyframeEffectModel.cpp b/Source/core/animation/KeyframeEffectModel.cpp
index 33f66270f3a46ab72c5a576ef831393bdf1dc452..0a3da176e4a676de16963872151b5a0ecd7d6974 100644
--- a/Source/core/animation/KeyframeEffectModel.cpp
+++ b/Source/core/animation/KeyframeEffectModel.cpp
@@ -268,6 +268,19 @@ void KeyframeEffectModel::ensureKeyframeGroups() const
     }
 }
 
+bool KeyframeEffectModel::isReplaceOnly()
+{
+    ensureKeyframeGroups();
+    for (KeyframeGroupMap::iterator iter = m_keyframeGroups->begin(); iter != m_keyframeGroups->end(); ++iter) {
+        const PropertySpecificKeyframeVector& keyframeVector = iter->value->keyframes();
+        for (size_t i = 0; i < keyframeVector.size(); ++i) {
+            if (keyframeVector[i]->value()->dependsOnUnderlyingValue()) {
+                return false;
+            }
+        }
+    }
+    return true;
+}
 
 KeyframeEffectModel::PropertySpecificKeyframe::PropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, const AnimatableValue* value, CompositeOperation composite)
     : m_offset(offset)