[import] support document.stylesheets in imported documents.

Source/core/dom/StyleSheetCandidate.cpp

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 13 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "core/dom/StyleSheetCandidate.h"
 
 #include "HTMLNames.h"
 #include "core/dom/Element.h"
 #include "core/dom/ProcessingInstruction.h"
 #include "core/dom/StyleEngine.h"
+#include "core/html/HTMLImport.h"
 #include "core/html/HTMLLinkElement.h"
 #include "core/html/HTMLStyleElement.h"
 #include "core/svg/SVGStyleElement.h"
 
 namespace WebCore {
 
 using namespace HTMLNames;
 
 AtomicString StyleSheetCandidate::title() const
 {
     return isElement() ? toElement(m_node).getAttribute(titleAttr) : nullAtom;
 }
 
 bool StyleSheetCandidate::isXSL() const
 {
     return !m_node.document().isHTMLDocument() && m_type == Pi && toProcessingInstruction(m_node).isXSL();
 }
 
 bool StyleSheetCandidate::isImport() const
 {
     return m_type == HTMLLink && toHTMLLinkElement(m_node).isImport();
 }
 
 Document* StyleSheetCandidate::importedDocument() const
 {
     ASSERT(isImport());
-    return toHTMLLinkElement(m_node).import();
+    // The stylesheet update traversal shouldn't go into shared import
+    // to prevent it from stepping into cycle.
+    HTMLLinkElement& element = toHTMLLinkElement(m_node);
+    if (!element.importOwnsLoader())
+        return 0;
+    return element.import();
 }
 
 bool StyleSheetCandidate::isAlternate() const
 {
     if (!isElement())
         return false;
     return toElement(m_node).getAttribute(relAttr).contains("alternate");
 }
 
 bool StyleSheetCandidate::isEnabledViaScript() const
@@ skipping 59 matching lines @@
         return toSVGStyleElement(m_node).sheet();
     case Pi:
         return toProcessingInstruction(m_node).sheet();
     }
 
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 }