headless: Support --use-gl in headless shell

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 4a62454cc46fdc29401fa6746af400831d1da648..9b40f28abe5bf7904e07437150dd82e1071d8fc5 100644
--- a/headless/app/headless_shell_switches.cc
+++ b/headless/app/headless_shell_switches.cc
@@ -27,5 +27,9 @@ const char kRepl[] = "repl";
 // Save a screenshot of the loaded page.
 const char kScreenshot[] = "screenshot";
 
+// Sets the GL implementation to use. Use a blank string to disable GL
+// rendering.
+const char kUseGL[] = "use-gl";
+
 }  // namespace switches
 }  // namespace headless