Fix crash in CSS.getLayoutTreeAndStyles

third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp

 /*
  * Copyright (C) 2010, 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:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 2351 matching lines @@
         visitLayoutTreeNodes(&elementShadow->youngestShadowRoot(),
                              layoutTreeNodes, cssPropertyWhitelist,
                              styleToIndexMap, computedStyles);
       }
     }
 
     // Pierce iframe boundaries.
     if (node->isFrameOwnerElement()) {
       Document* contentDocument =
           toHTMLFrameOwnerElement(node)->contentDocument();
-      contentDocument->updateStyleAndLayoutTree();
-      visitLayoutTreeNodes(contentDocument->documentElement(), layoutTreeNodes,
-                           cssPropertyWhitelist, styleToIndexMap,
-                           computedStyles);
+      if (contentDocument) {
+        contentDocument->updateStyleAndLayoutTree();
+        visitLayoutTreeNodes(contentDocument->documentElement(),
+                             layoutTreeNodes, cssPropertyWhitelist,
+                             styleToIndexMap, computedStyles);
+      }
     }
 
     LayoutObject* layoutObject = node->layoutObject();
     if (!layoutObject)
       continue;
 
     int nodeId = m_domAgent->boundNodeId(node);
     if (!nodeId)
       continue;
 
@@ skipping 128 matching lines @@
   visitor->trace(m_cssStyleSheetToInspectorStyleSheet);
   visitor->trace(m_documentToCSSStyleSheets);
   visitor->trace(m_invalidatedDocuments);
   visitor->trace(m_nodeToInspectorStyleSheet);
   visitor->trace(m_inspectorUserAgentStyleSheet);
   visitor->trace(m_tracker);
   InspectorBaseAgent::trace(visitor);
 }
 
 }  // namespace blink