Copy m_namespaceRules when copying StyleSheetContents to avoid crashing.

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

Index: third_party/WebKit/Source/core/css/StyleRuleNamespace.h
diff --git a/third_party/WebKit/Source/core/css/StyleRuleNamespace.h b/third_party/WebKit/Source/core/css/StyleRuleNamespace.h
index dd3d031c60c4a80ed5e1b9c3ed2e936d286ba46a..ada5d52c4c1f5516f3b0a21b29037873a1428d05 100644
--- a/third_party/WebKit/Source/core/css/StyleRuleNamespace.h
+++ b/third_party/WebKit/Source/core/css/StyleRuleNamespace.h
@@ -17,8 +17,15 @@ class StyleRuleNamespace final : public StyleRuleBase {
     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);