Copy m_namespaceRules when copying StyleSheetContents to avoid crashing.

third_party/WebKit/Source/core/css/StyleRule.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2005, 2006, 2008, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 132 matching lines @@
     case Supports:
       return toStyleRuleSupports(this)->copy();
     case Import:
       // FIXME: Copy import rules.
       ASSERT_NOT_REACHED();
       return nullptr;
     case Keyframes:
       return toStyleRuleKeyframes(this)->copy();
     case Viewport:
       return toStyleRuleViewport(this)->copy();
+    case Namespace:
+      return toStyleRuleNamespace(this)->copy();
     case Charset:
     case Keyframe:
-    case Namespace:
       ASSERT_NOT_REACHED();
       return nullptr;
   }
   ASSERT_NOT_REACHED();
   return nullptr;
 }
 
 CSSRule* StyleRuleBase::createCSSOMWrapper(CSSStyleSheet* parentSheet,
                                            CSSRule* parentRule) const {
   CSSRule* rule = nullptr;
@@ skipping 214 matching lines @@
     m_properties = m_properties->mutableCopy();
   return *toMutableStylePropertySet(m_properties);
 }
 
 DEFINE_TRACE_AFTER_DISPATCH(StyleRuleViewport) {
   visitor->trace(m_properties);
   StyleRuleBase::traceAfterDispatch(visitor);
 }
 
 }  // namespace blink