Have Element::ensureMutableInlineStyle() return a reference

Source/core/css/CSSKeyframeRule.cpp

Index: Source/core/css/CSSKeyframeRule.cpp
diff --git a/Source/core/css/CSSKeyframeRule.cpp b/Source/core/css/CSSKeyframeRule.cpp
index a49ef8e9508d8ce697e9aa277c02342dfcd9b1f9..20e1e4755ae23831778d4db1c2b4146fcad20036 100644
--- a/Source/core/css/CSSKeyframeRule.cpp
+++ b/Source/core/css/CSSKeyframeRule.cpp
@@ -92,15 +92,16 @@ void StyleKeyframe::setKeys(PassOwnPtr<Vector<double> > keys)
     ASSERT(m_keyText.isNull());
 }
 
-MutableStylePropertySet* StyleKeyframe::mutableProperties()
+MutableStylePropertySet& StyleKeyframe::mutableProperties()
 {
     if (!m_properties->isMutable())
         m_properties = m_properties->mutableCopy();
-    return toMutableStylePropertySet(m_properties);
+    return *toMutableStylePropertySet(m_properties);
 }
 
 void StyleKeyframe::setProperties(PassRefPtr<StylePropertySet> properties)
 {
+    ASSERT(properties);
     m_properties = properties;
 }