Have Element::ensureMutableInlineStyle() return a reference

Source/core/css/CSSKeyframeRule.cpp

 /*
  * Copyright (C) 2007, 2008, 2012 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
@@ skipping 74 matching lines @@
 }
 
 void StyleKeyframe::setKeys(PassOwnPtr<Vector<double> > keys)
 {
     ASSERT(keys && !keys->isEmpty());
     m_keys = keys;
     m_keyText = String();
     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;
 }
 
 String StyleKeyframe::cssText() const
 {
     StringBuilder result;
     result.append(keyText());
     result.appendLiteral(" { ");
     String decls = m_properties->asText();
     result.append(decls);
@@ skipping 42 matching lines @@
     return m_propertiesCSSOMWrapper.get();
 }
 
 void CSSKeyframeRule::reattach(StyleRuleBase*)
 {
     // No need to reattach, the underlying data is shareable on mutation.
     ASSERT_NOT_REACHED();
 }
 
 } // namespace WebCore