Bug 1571092 - Add missing return statement. r=jld

Differential Revision: https://phabricator.services.mozilla.com/D40527

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

ipc/glue/GeckoChildProcessHost.cpp

@@ -1445,7 +1445,7 @@ RefPtr<ProcessHandlePromise> WindowsProcessLauncher::DoLaunch() {
                  mCmdLine->command_line_string().c_str());
       return ProcessHandlePromise::CreateAndResolve(handle, __func__);
     }
-    ProcessHandlePromise::CreateAndReject(LaunchError{}, __func__);
+    return ProcessHandlePromise::CreateAndReject(LaunchError{}, __func__);
   }
 #  endif  // defined(MOZ_SANDBOX)