[devtools] Add DOMSnapshot domain for dom+layout+style snapshots.

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 e1fea63aac13cc0c1b89c0460a58943a5a043df8..65d51d3846818bc299d5e0f4dfbf6b4bad07e8ca 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
+++ b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.cpp
@@ -2308,7 +2308,11 @@ Response InspectorCSSAgent::getLayoutTreeAndStyles(
         layout_tree_nodes,
     std::unique_ptr<protocol::Array<protocol::CSS::ComputedStyle>>*
         computed_styles) {
-  dom_agent_->GetDocument()->UpdateStyleAndLayoutTree();
+  Document* document = dom_agent_->GetDocument();
+  if (!document)
+    return Response::Error("Document is not available");
+
+  document->UpdateStyleAndLayoutTree();
 
   // Look up the CSSPropertyIDs for each entry in |styleWhitelist|.
   Vector<std::pair<String, CSSPropertyID>> css_property_whitelist;