Implement a runtime headless mode for Linux

ui/aura/env.cc

Index: ui/aura/env.cc
diff --git a/ui/aura/env.cc b/ui/aura/env.cc
index d4b5cdfe43d0d7914753efd2975cba71acaa66db..5b7fe69a5a574aac949fed48f97321ce397ecbac 100644
--- a/ui/aura/env.cc
+++ b/ui/aura/env.cc
@@ -18,6 +18,7 @@
 #include "ui/aura/window_port_local.h"
 #include "ui/events/event_target_iterator.h"
 #include "ui/events/platform/platform_event_source.h"
+#include "ui/gfx/switches.h"
 
 #if defined(USE_OZONE)
 #include "ui/ozone/public/ozone_platform.h"
@@ -37,6 +38,10 @@ bool RunningInsideMus() {
       "primordial-pipe-token");
 }
 
+bool RunningInHeadlessMode() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kHeadless);
+}
+
 }  // namespace
 
 // Observes destruction and changes of the FocusClient for a window.
@@ -162,7 +167,7 @@ Env::Env(Mode mode)
 }
 
 void Env::Init() {
-  if (RunningInsideMus())
+  if (RunningInsideMus() || RunningInHeadlessMode())
     return;
 #if defined(USE_OZONE)
   // The ozone platform can provide its own event source. So initialize the