headless: Support --use-gl in headless shell

headless/app/headless_shell.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 <iostream>
 #include <memory>
 #include <string>
 
 #include "base/base64.h"
 #include "base/bind.h"
@@ skipping 316 matching lines @@
       return EXIT_FAILURE;
     }
     builder.SetProxyServer(parsed_proxy_server);
   }
 
   if (command_line.HasSwitch(switches::kHostResolverRules)) {
     builder.SetHostResolverRules(
         command_line.GetSwitchValueASCII(switches::kHostResolverRules));
   }
 
+  if (command_line.HasSwitch(headless::switches::kUseGL)) {
+    builder.SetGLImplementation(
+        command_line.GetSwitchValueASCII(headless::switches::kUseGL));
+  }
+
   return HeadlessBrowserMain(
       builder.Build(),
       base::Bind(&HeadlessShell::OnStart, base::Unretained(&shell)));
 }