Copy m_namespaceRules when copying StyleSheetContents to avoid crashing.

third_party/WebKit/Source/core/css/StyleRuleNamespace.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef StyleRuleNamespace_h
 #define StyleRuleNamespace_h
 
 #include "core/css/StyleRule.h"
 
 namespace blink {
 
 // This class is never actually stored anywhere currently, but only used for
 // the parser to pass to a stylesheet
 class StyleRuleNamespace final : public StyleRuleBase {
  public:
   static StyleRuleNamespace* create(AtomicString prefix, AtomicString uri) {
     return new StyleRuleNamespace(prefix, uri);
   }
 
+  StyleRuleNamespace* copy() const {
+    return new StyleRuleNamespace(m_prefix, m_uri);
+  }
+
   AtomicString prefix() const { return m_prefix; }
-  AtomicString uri() const { return m_uri; }
+  AtomicString uri() const {
+    DCHECK(!m_uri.isHashTableDeletedValue());
+    return m_uri;
+  }
 
   DEFINE_INLINE_TRACE_AFTER_DISPATCH() {
     StyleRuleBase::traceAfterDispatch(visitor);
   }
 
  private:
   StyleRuleNamespace(AtomicString prefix, AtomicString uri)
       : StyleRuleBase(Namespace), m_prefix(prefix), m_uri(uri) {}
 
   AtomicString m_prefix;
   AtomicString m_uri;
 };
 
 DEFINE_STYLE_RULE_TYPE_CASTS(Namespace);
 
 }  // namespace blink
 
 #endif  // StyleRuleNamespace_h