Use Proxy to provide protocol APIs

In this way we can avoid initializing defaultSession when protocol
module is used.

lib/browser/api/protocol.js

@@ -1,17 +1,19 @@
-const {app, session} = require('electron')
-const {registerStandardSchemes} = process.atomBinding('protocol')
+const {session} = require('electron')
 
-exports.registerStandardSchemes = registerStandardSchemes
+// Global protocol APIs.
+module.exports = process.atomBinding('protocol')
 
-const setupProtocol = function () {
-  let protocol = session.defaultSession.protocol
-  for (let method in protocol) {
-    exports[method] = protocol[method].bind(protocol)
+// Fallback protocol APIs of default session.
+Object.setPrototypeOf(module.exports, new Proxy({}, {
+  get (target, property) {
+    return (...args) => session.defaultSession.protocol[property](...args)
+  },
+
+  ownKeys () {
+    return Object.getOwnPropertyNames(session.defaultSession.protocol)
+  },
+
+  getOwnPropertyDescriptor (target) {
+    return { configurable: true, enumerable: true }
   }
-}
-
-if (app.isReady()) {
-  setupProtocol()
-} else {
-  app.once('ready', setupProtocol)
-}
+}))