Use the marionette reftest implemenation on Windows

Differential Revision: https://phabricator.services.mozilla.com/D5519

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1490272
gecko-commit: 4dfb6e690645619a71da734ee66ea658a1ef253a
gecko-integration-branch: mozilla-inbound
gecko-reviewers: ato
diff --git a/tools/wptrunner/wptrunner/wptcommandline.py b/tools/wptrunner/wptrunner/wptcommandline.py
index 56f2c36..d6d905a 100644
--- a/tools/wptrunner/wptrunner/wptcommandline.py
+++ b/tools/wptrunner/wptrunner/wptcommandline.py
@@ -259,7 +259,7 @@
     gecko_group.add_argument("--stylo-threads", action="store", type=int, default=1,
                              help="Number of parallel threads to use for stylo")
     gecko_group.add_argument("--reftest-internal", dest="reftest_internal", action="store_true",
-                             default=None, help="Enable reftest runner implemented inside Marionette")
+                             default=True, help="Enable reftest runner implemented inside Marionette")
     gecko_group.add_argument("--reftest-external", dest="reftest_internal", action="store_false",
                              help="Disable reftest runner implemented inside Marionette")
     gecko_group.add_argument("--reftest-screenshot", dest="reftest_screenshot", action="store",
@@ -512,10 +512,6 @@
         kwargs['extra_prefs'] = [tuple(prefarg.split('=', 1)) for prefarg in
                                  kwargs['extra_prefs']]
 
-    if kwargs["reftest_internal"] is None:
-        # Default to the internal reftest implementation on Linux and OSX
-        kwargs["reftest_internal"] = sys.platform.startswith("linux") or sys.platform.startswith("darwin")
-
     return kwargs