Bug 1603016 - Add a --headless option for mach reftest. r=dbaron

Differential Revision: https://phabricator.services.mozilla.com/D56663

--HG--
extra : moz-landing-system : lando

layout/tools/reftest/reftestcommandline.py

@@ -260,6 +260,12 @@ class ReftestArgumentsParser(argparse.ArgumentParser):
                           default=False,
                           help="Enable the WebRender compositor in Gecko.")
 
+        self.add_argument("--headless",
+                          action="store_true",
+                          dest="headless",
+                          default=False,
+                          help="Run tests in headless mode.")
+
         mozlog.commandline.add_logging_group(self)
 
     def get_ip(self):

layout/tools/reftest/runreftest.py

@@ -468,6 +468,10 @@ class RefTest(object):
             browserEnv["MOZ_ACCELERATED"] = "1"
         else:
             browserEnv["MOZ_WEBRENDER"] = "0"
+
+        if options.headless:
+            browserEnv["MOZ_HEADLESS"] = "1"
+
         return browserEnv
 
     def cleanup(self, profileDir):