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

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

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * 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
@@ skipping 458 matching lines @@
                              pierce.fromMaybe(false),
                              document_node_to_id_map_.Get());
   return Response::OK();
 }
 
 Response InspectorDOMAgent::getFlattenedDocument(
     Maybe<int> depth,
     Maybe<bool> pierce,
     std::unique_ptr<protocol::Array<protocol::DOM::Node>>* nodes) {
   if (!Enabled())
-    return Response::Error("Document not enabled");
+    return Response::Error("DOM agent hasn't been enabled");
 
   if (!document_)
     return Response::Error("Document is not available");
 
   DiscardFrontendBindings();
 
   int sanitized_depth = depth.fromMaybe(-1);
   if (sanitized_depth == -1)
     sanitized_depth = INT_MAX;
 
@@ skipping 1674 matching lines @@
   visitor->Trace(id_to_nodes_map_);
   visitor->Trace(document_);
   visitor->Trace(revalidate_task_);
   visitor->Trace(search_results_);
   visitor->Trace(history_);
   visitor->Trace(dom_editor_);
   InspectorBaseAgent::Trace(visitor);
 }
 
 }  // namespace blink