headless: Support --use-gl in headless shell

headless/app/headless_shell.cc

Index: headless/app/headless_shell.cc
diff --git a/headless/app/headless_shell.cc b/headless/app/headless_shell.cc
index f9e8e3f45d4ecc6292458fe1f135f797bb7b7c0a..a3e0b7435f6dbd5bc48bfba297f9df45501c68e8 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -334,6 +334,11 @@ int main(int argc, const char** argv) {
         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)));