Add window.loadFile and webContents.loadFile helper methods

lib/browser/api/web-contents.js

@@ -2,6 +2,8 @@
 
 const {EventEmitter} = require('events')
 const electron = require('electron')
+const path = require('path')
+const url = require('url')
 const {app, ipcMain, session, NavigationController} = electron
 
 // session is not used here, the purpose is to make sure session is initalized
@@ -243,6 +245,17 @@ WebContents.prototype.getZoomLevel = function (callback) {
   })
 }
 
+WebContents.prototype.loadFile = function (filePath) {
+  if (typeof filePath !== 'string') {
+    throw new Error('Must pass filePath as a string')
+  }
+  return this.loadURL(url.format({
+    protocol: 'file',
+    slashes: true,
+    pathname: path.resolve(app.getAppPath(), filePath)
+  }))
+}
+
 WebContents.prototype.getZoomFactor = function (callback) {
   if (typeof callback !== 'function') {
     throw new Error('Must pass function as an argument')