gl_surface_egl: ensure off and on screen surfaces use the same EGLConfig

ui/gfx/x/x11_switches.cc

Index: ui/gfx/x/x11_switches.cc
diff --git a/ui/gfx/x/x11_switches.cc b/ui/gfx/x/x11_switches.cc
index bda0ad3c5572800306fafa4fb9073db46393074b..ba7f1bdb724498ad8e882a8bacffe40488266680 100644
--- a/ui/gfx/x/x11_switches.cc
+++ b/ui/gfx/x/x11_switches.cc
@@ -7,6 +7,13 @@
 namespace switches {
 
 #if !defined(OS_CHROMEOS)
+// When enabled, tries to get a transparent X11 visual so that we can have
+// per-pixel alpha in windows.
+//
+// TODO(erg): Remove this switch once we've stabilized the code
+// path. http://crbug.com/369209
+const char kEnableTransparentVisuals[] = "enable-transparent-visuals";
+
 // Which X11 display to connect to. Emulates the GTK+ "--display=" command line
 // argument.
 const char kX11Display[] = "display";