Copy m_namespaceRules when copying StyleSheetContents to avoid crashing.

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

Index: third_party/WebKit/Source/core/css/StyleRule.cpp
diff --git a/third_party/WebKit/Source/core/css/StyleRule.cpp b/third_party/WebKit/Source/core/css/StyleRule.cpp
index 9bb6e3e458a6d2bb53570b18b3f9ccfb92c2b6f0..dc19a0ee84385761a606599d5836d04e74188e8f 100644
--- a/third_party/WebKit/Source/core/css/StyleRule.cpp
+++ b/third_party/WebKit/Source/core/css/StyleRule.cpp
@@ -150,9 +150,10 @@ StyleRuleBase* StyleRuleBase::copy() const {
       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;
   }