Bug 1494187 - Run the manifest download steps before trying to update the manifest, r=ato

This ensures that we always start from a partial manifest where possible and also ensures that the
configuration files are correctly created (a refactor to create these irrespective of whether we
do a download would make sense, but this fixes the immediate problem)

Depends on D7088

Differential Revision: https://phabricator.services.mozilla.com/D7089

testing/web-platform/mach_commands.py

@@ -435,6 +435,7 @@ class MachCommands(MachCommandBase):
              parser=create_parser_manifest_update)
     def wpt_manifest_update(self, **params):
         self.setup()
+        self.wpt_manifest_download(**params)
         wpt_manifest_updater = self._spawn(WPTManifestUpdater)
         return wpt_manifest_updater.run_update(**params)