CSSValue Immediates: Replace CSSPrimitiveValue usage with const references

Source/core/animation/StringKeyframe.cpp

Index: Source/core/animation/StringKeyframe.cpp
diff --git a/Source/core/animation/StringKeyframe.cpp b/Source/core/animation/StringKeyframe.cpp
index 048cd017b8a61e72ea3eb20cc33057b32c1d30f1..ea2cf6daead2544a4b660b53e3688d18f751db76 100644
--- a/Source/core/animation/StringKeyframe.cpp
+++ b/Source/core/animation/StringKeyframe.cpp
@@ -106,12 +106,12 @@ DEFINE_TRACE(StringKeyframe)
     Keyframe::trace(visitor);
 }
 
-StringKeyframe::CSSPropertySpecificKeyframe::CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, NullableCSSValue value, EffectModel::CompositeOperation op)
+StringKeyframe::CSSPropertySpecificKeyframe::CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, const NullableCSSValue& value, EffectModel::CompositeOperation op)
     : Keyframe::PropertySpecificKeyframe(offset, easing, op)
     , m_value(value)
 { }
 
-StringKeyframe::CSSPropertySpecificKeyframe::CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, NullableCSSValue value)
+StringKeyframe::CSSPropertySpecificKeyframe::CSSPropertySpecificKeyframe(double offset, PassRefPtr<TimingFunction> easing, const NullableCSSValue& value)
     : Keyframe::PropertySpecificKeyframe(offset, easing, EffectModel::CompositeReplace)
     , m_value(value)
 {
@@ -194,8 +194,8 @@ PassRefPtrWillBeRawPtr<Interpolation> StringKeyframe::CSSPropertySpecificKeyfram
     // FIXME: Remove this check once neutral keyframes are implemented in StringKeyframes.
     if (!nullableFromCSSValue || !nullableToCSSValue)
         return DeferredLegacyStyleInterpolation::create(nullableFromCSSValue, nullableToCSSValue, property);
-    CSSValue& fromCSSValue = *nullableFromCSSValue;
-    CSSValue& toCSSValue = *nullableToCSSValue;
+    const CSSValue& fromCSSValue = *nullableFromCSSValue;
+    const CSSValue& toCSSValue = *nullableToCSSValue;
 
     if (!CSSPropertyMetadata::isInterpolableProperty(property)) {
         if (fromCSSValue.ptrEquals(toCSSValue))