add print to pdf for headless

content/public/browser/devtools_manager_delegate.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_DEVTOOLS_MANAGER_DELEGATE_H_
 #define CONTENT_PUBLIC_BROWSER_DEVTOOLS_MANAGER_DELEGATE_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 33 matching lines @@
   // Creates new inspectable target given the |url|.
   virtual scoped_refptr<DevToolsAgentHost> CreateNewTarget(const GURL& url);
 
   // Result ownership is passed to the caller.
   virtual base::DictionaryValue* HandleCommand(
       DevToolsAgentHost* agent_host,
       base::DictionaryValue* command);
 
   using CommandCallback =
       base::Callback<void(std::unique_ptr<base::DictionaryValue> response)>;
+  // Handle async command, feed response to CommandCallback when it is ready.
   virtual bool HandleAsyncCommand(DevToolsAgentHost* agent_host,
                                   base::DictionaryValue* command,
-                                  CommandCallback callback);
+                                  const CommandCallback& callback);
+
   // Should return discovery page HTML that should list available tabs
   // and provide attach links.
   virtual std::string GetDiscoveryPageHTML();
 
   // Returns frontend resource data by |path|.
   virtual std::string GetFrontendResource(const std::string& path);
 
   virtual ~DevToolsManagerDelegate();
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_DEVTOOLS_MANAGER_DELEGATE_H_