Fix crash in CSS.getLayoutTreeAndStyles

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

Index: third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
diff --git a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
index 7d71f1b9105d8de85a088945826cb2b7e214565c..a9cece0fac4142bcbf56fc0013e133c534711868 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
+++ b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
@@ -2369,10 +2369,12 @@ void InspectorCSSAgent::visitLayoutTreeNodes(
     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();