Adds a command-line flag indicating an open and listening socket to

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 {
 
 // The directory breakpad should store minidumps in.
@@ skipping 13 matching lines @@
 // 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";
 
+// The given value is the fd of a socket already opened by the parent process.
+// This allows automation to provide a listening socket for clients to connect
+// to before chrome is fully fired up. In particular, a parent process can
+// open the port, exec headles chrome, and connect to the devtools port
+// immediately. Waiting for chrome to be ready is then handled by the first
+// read from the port, which will block until chrome is ready. No polling is
+// needed.
+const char kRemoteDebuggingSocketFd[] = "remote-debugging-socket-fd";
+
 // 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";
 
 // Issues a stop after the specified number of milliseconds.  This cancels all
 // navigation and causes the DOMContentLoaded event to fire.
 const char kTimeout[] = "timeout";
@@ skipping 11 matching lines @@
 // network fetches have completed, timers fire and if the system runs out of
 // virtual time is fastforwarded so the next timer fires immediatley, until the
 // specified virtual time budget is exhausted.
 const char kVirtualTimeBudget[] = "virtual-time-budget";
 
 // Sets the initial window size. Provided as string in the format "800,600".
 const char kWindowSize[] = "window-size";
 
 }  // namespace switches
 }  // namespace headless