Make --headless flag work on Mac

headless/lib/browser/headless_content_browser_client.cc

Index: headless/lib/browser/headless_content_browser_client.cc
diff --git a/headless/lib/browser/headless_content_browser_client.cc b/headless/lib/browser/headless_content_browser_client.cc
index ade0c60f0ba2b403e4d39ab1236135b68086da06..5876199424bb691c0a56ce85a1004afc8f75f896 100644
--- a/headless/lib/browser/headless_content_browser_client.cc
+++ b/headless/lib/browser/headless_content_browser_client.cc
@@ -28,6 +28,7 @@
 #include "headless/lib/headless_macros.h"
 #include "storage/browser/quota/quota_settings.h"
 #include "ui/base/resource/resource_bundle.h"
+#include "ui/gfx/switches.h"
 
 #if defined(HEADLESS_USE_BREAKPAD)
 #include "base/debug/leak_annotations.h"
@@ -186,6 +187,7 @@ void HeadlessContentBrowserClient::GetAdditionalMappedFilesForChildProcess(
 void HeadlessContentBrowserClient::AppendExtraCommandLineSwitches(
     base::CommandLine* command_line,
     int child_process_id) {
+  command_line->AppendSwitch(switches::kHeadless);
 #if defined(HEADLESS_USE_BREAKPAD)
   // This flag tells child processes to also turn on crash reporting.
   if (breakpad::IsCrashReporterEnabled())