linux_aura: Add a flag for experimenting with ARGB windows.

ui/views/views_switches.cc

Index: ui/views/views_switches.cc
diff --git a/ui/views/views_switches.cc b/ui/views/views_switches.cc
index c62e83d1d8bcfd304974a480bad22343b64198b2..73e3a6f6b217e91b598ca16106ab3fb7825b9132 100644
--- a/ui/views/views_switches.cc
+++ b/ui/views/views_switches.cc
@@ -16,6 +16,13 @@ namespace switches {
 const char kDisableViewsRectBasedTargeting[] =
     "disable-views-rect-based-targeting";
 
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+// When enabled, tries to get a transparent X11 visual so that we can have
+// per-pixel alpha in windows. To be removed once we've stabilized the code
+// path.
+const char kEnableTransparentVisuals[] = "enable-transparent-visuals";
+#endif
+
 bool IsRectBasedTargetingEnabled() {
 #if defined(OS_CHROMEOS) || defined(OS_WIN)
   return !CommandLine::ForCurrentProcess()->