CSSValue Immediates: Replace CSSPrimitiveValue usage with const references

Source/core/animation/CSSValueInterpolationType.h

Index: Source/core/animation/CSSValueInterpolationType.h
diff --git a/Source/core/animation/CSSValueInterpolationType.h b/Source/core/animation/CSSValueInterpolationType.h
index f07ebfa49fb7ebb3d4246db92c0444d670019948..329ec4e8e780f0f7192aadc1363fea5908bf08d7 100644
--- a/Source/core/animation/CSSValueInterpolationType.h
+++ b/Source/core/animation/CSSValueInterpolationType.h
@@ -24,12 +24,12 @@ public:
 class DefaultNonInterpolableValue : public NonInterpolableValue {
 public:
     virtual ~DefaultNonInterpolableValue() { }
-    static PassRefPtrWillBeRawPtr<DefaultNonInterpolableValue> create(NullableCSSValue cssValue)
+    static PassRefPtrWillBeRawPtr<DefaultNonInterpolableValue> create(const NullableCSSValue& cssValue)
     {
         return adoptRefWillBeNoop(new DefaultNonInterpolableValue(cssValue));
     }
 
-    NullableCSSValue cssValue() const { return m_cssValue; }
+    const NullableCSSValue& cssValue() const { return m_cssValue; }
 
     DEFINE_INLINE_VIRTUAL_TRACE()
     {
@@ -40,7 +40,7 @@ public:
     DECLARE_NON_INTERPOLABLE_VALUE_TYPE();
 
 private:
-    DefaultNonInterpolableValue(NullableCSSValue cssValue)
+    DefaultNonInterpolableValue(const NullableCSSValue& cssValue)
         : m_cssValue(cssValue)
     { }