Add user-agent flag to headless

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 42 matching lines @@
 // deeming the page to be ready.  For determinism virtual time does not advance
 // while there are pending network fetches (i.e no timers will fire). Once all
 // 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";
 
+// A string used to override the default user agent with a custom one.
+const char kUserAgent[] = "user-agent";
+
 }  // namespace switches
 }  // namespace headless