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

headless/app/headless_shell_switches.cc

Index: headless/app/headless_shell_switches.cc
diff --git a/headless/app/headless_shell_switches.cc b/headless/app/headless_shell_switches.cc
index 86ced8127b5fb7f3ad8e6ae82d8e9f439b041573..9cf9baa4e3393b9ba16f1a18461f355b67331e89 100644
--- a/headless/app/headless_shell_switches.cc
+++ b/headless/app/headless_shell_switches.cc
@@ -39,6 +39,15 @@ const char kProxyServer[] = "proxy-server";
 // 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";