Setup Relay backend in Electron shell

This avoids an "unknown call" warning (yellow box).

Author: matthewwithanm

shells/electron/src/backend.js

@@ -30,6 +30,7 @@ var Agent = require('../../../agent/Agent');
 var Bridge = require('../../../agent/Bridge');
 var inject = require('../../../agent/inject');
 var setupRNStyle = require('../../../plugins/ReactNativeStyle/setupBackend');
+var setupRelay = require('../../../plugins/Relay/backend');
 
 FOR_BACKEND.wall.onClose(() => {
  if (agent) {
@@ -50,6 +51,8 @@ if (FOR_BACKEND.resolveRNStyle) {
  setupRNStyle(bridge, agent, FOR_BACKEND.resolveRNStyle);
 }
 
+setupRelay(bridge, agent, window.__REACT_DEVTOOLS_GLOBAL_HOOK__);
+
 var _connectTimeout = setTimeout(() => {
  console.warn('react-devtools agent got no connection');
 }, 20000);