Have Element::ensureMutableInlineStyle() return a reference

Source/core/css/CSSKeyframeRule.h

Index: Source/core/css/CSSKeyframeRule.h
diff --git a/Source/core/css/CSSKeyframeRule.h b/Source/core/css/CSSKeyframeRule.h
index a32a4057d255a2e103395243d1459f4692a643d8..31d78d3a845109af695226311a066b3255a71a97 100644
--- a/Source/core/css/CSSKeyframeRule.h
+++ b/Source/core/css/CSSKeyframeRule.h
@@ -55,8 +55,8 @@ public:
     // Used by BisonCSSParser when constructing a new StyleKeyframe.
     void setKeys(PassOwnPtr<Vector<double> >);
 
-    const StylePropertySet* properties() const { return m_properties.get(); }
-    MutableStylePropertySet* mutableProperties();
+    const StylePropertySet& properties() const { return *m_properties; }
+    MutableStylePropertySet& mutableProperties();
     void setProperties(PassRefPtr<StylePropertySet>);
 
     String cssText() const;