[content shell] run headless when in layout test mode

content/shell/shell_switches.cc

Index: content/shell/shell_switches.cc
diff --git a/content/shell/shell_switches.cc b/content/shell/shell_switches.cc
index be7f7cddd5a8c3c0c56eb70788dc0eab49353820..df50d5e28063132381cfc25d6142a7ae7b83e1f8 100644
--- a/content/shell/shell_switches.cc
+++ b/content/shell/shell_switches.cc
@@ -27,4 +27,7 @@ const char kNoTimeout[] = "no-timeout";
 // Save results when layout-as-browser tests fail.
 const char kOutputLayoutTestDifferences[] = "output-layout-test-differences";
 
+// Show the content_shell window, even when running in layout test mode.
+const char kShowContentShell[] = "show-content-shell";
+
 }  // namespace switches