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..3a49c7cec6de1a60d773e235e9bd336154373980 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.
@@ -170,6 +175,8 @@ void Env::Init() {
   // let the mus process initialize ozone instead.
   ui::OzonePlatform::InitializeForUI();
 #endif
+  if (RunningInHeadlessMode())
+    return;
   if (!ui::PlatformEventSource::GetInstance())
     event_source_ = ui::PlatformEventSource::CreateDefault();
 }