headless: Implement screenshot capturing

headless/app/headless_shell_switches.cc

 // Copyright 2016 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.
 
 #include "headless/app/headless_shell_switches.h"
 
 namespace headless {
 namespace switches {
 
 // Instructs headless_shell to print document.body.innerHTML to stdout.
 const char kDumpDom[] = "dump-dom";
 
 // Uses a specified proxy server, overrides system settings. This switch only
 // affects HTTP and HTTPS requests.
 const char kProxyServer[] = "proxy-server";
 
 // Use the given address instead of the default loopback for accepting remote
 // debugging connections. Should be used together with --remote-debugging-port.
 // Note that the remote debugging protocol does not perform any authentication,
 // so exposing it too widely can be a security risk.
 const char kRemoteDebuggingAddress[] = "remote-debugging-address";
 
 // Runs a read-eval-print loop that allows the user to evaluate Javascript
 // expressions.
 const char kRepl[] = "repl";
 
+// Save a screenshot of the loaded page.
+const char kScreenshot[] = "screenshot";
+
 }  // namespace switches
 }  // namespace headless