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diff --git a/Source/WebCore/rendering/style/WillChangeData.h b/Source/WebCore/rendering/style/WillChangeData.h
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+++ b/Source/WebCore/rendering/style/WillChangeData.h
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+/*
+ * Copyright (C) 2015 Apple 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:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "CSSPropertyNames.h"
+#include "RenderStyleConstants.h"
+#include <wtf/RefCounted.h>
+#include <wtf/Vector.h>
+
+namespace WebCore {
+
+class WillChangeData : public RefCounted<WillChangeData> {
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    static Ref<WillChangeData> create()
+    {
+        return adoptRef(*new WillChangeData);
+    }
+    
+    bool operator==(const WillChangeData&) const;
+    bool operator!=(const WillChangeData& o) const
+    {
+        return !(*this == o);
+    }
+
+    bool isAuto() const { return m_animatableFeatures.isEmpty(); }
+    size_t numFeatures() const { return m_animatableFeatures.size(); }
+
+    bool containsScrollPosition() const;
+    bool containsContents() const;
+    bool containsProperty(CSSPropertyID) const;
+
+    bool canCreateStackingContext() const { return m_canCreateStackingContext; }
+    bool canTriggerCompositing() const { return m_canTriggerCompositing; }
+    bool canTriggerCompositingOnInline() const { return m_canTriggerCompositingOnInline; }
+
+    enum Feature {
+        ScrollPosition,
+        Contents,
+        Property,
+        Invalid
+    };
+
+    void addFeature(Feature, CSSPropertyID = CSSPropertyInvalid);
+    
+    typedef std::pair<Feature, CSSPropertyID> FeaturePropertyPair;
+    FeaturePropertyPair featureAt(size_t) const;
+
+    static bool propertyCreatesStackingContext(CSSPropertyID);
+
+private:
+    WillChangeData()
+    {
+    }
+
+    struct AnimatableFeature {
+        static const int numCSSPropertyIDBits = 14;
+        COMPILE_ASSERT(numCSSProperties < (1 << numCSSPropertyIDBits), CSSPropertyID_should_fit_in_14_bits);
+
+        unsigned m_feature : 2;
+        unsigned m_cssPropertyID : numCSSPropertyIDBits;
+
+        Feature feature() const
+        {
+            return static_cast<Feature>(m_feature);
+        }
+
+        CSSPropertyID property() const
+        {
+            return feature() == Property ? static_cast<CSSPropertyID>(m_cssPropertyID) : CSSPropertyInvalid;
+        }
+        
+        FeaturePropertyPair featurePropertyPair() const
+        {
+            return FeaturePropertyPair(feature(), property());
+        }
+
+        AnimatableFeature(Feature willChange, CSSPropertyID willChangeProperty = CSSPropertyInvalid)
+        {
+            switch (willChange) {
+            case Property:
+                ASSERT(willChangeProperty != CSSPropertyInvalid);
+                m_cssPropertyID = willChangeProperty;
+                FALLTHROUGH;
+            case ScrollPosition:
+            case Contents:
+                m_feature = static_cast<unsigned>(willChange);
+                break;
+            case Invalid:
+                ASSERT_NOT_REACHED();
+                break;
+            }
+        }
+        
+        bool operator==(const AnimatableFeature& other) const
+        {
+            return m_feature == other.m_feature && m_cssPropertyID == other.m_cssPropertyID;
+        }
+    };
+
+    Vector<AnimatableFeature, 1> m_animatableFeatures;
+    bool m_canCreateStackingContext { false };
+    bool m_canTriggerCompositing { false };
+    bool m_canTriggerCompositingOnInline { false };
+};
+
+} // namespace WebCore