Bug 1500001 - Fixup using non-firefox browsers via mach wpt, r=automatedtester

We need to pass in the extra kwargs that wpt run understands but
wptrunner does not. Ideally we could switch to using the run frontend
always and allow users to actually supply these kwargs, although that
might have some unwanted behaviour, and is a larger change. For now
this gets things working again.

Depends on D9081

Differential Revision: https://phabricator.services.mozilla.com/D9082

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

testing/web-platform/mach_commands.py

@@ -125,6 +125,12 @@ class WebPlatformTestsRunnerSetup(MozbuildObject):
 
         from tools.wpt import run
 
+        # Add additional kwargs consumed by the run frontend. Currently we don't
+        # have a way to set these through mach
+        kwargs["channel"] = None
+        kwargs["prompt"] = True
+        kwargs["install_browser"] = False
+
         try:
             kwargs = run.setup_wptrunner(run.virtualenv.Virtualenv(self.virtualenv_manager.virtualenv_root),
                                          **kwargs)