fix signature of zoom getter apis

lib/browser/api/web-contents.js

@@ -206,6 +206,26 @@ WebContents.prototype.printToPDF = function (options, callback) {
   this._printToPDF(printingSetting, callback)
 }
 
+WebContents.prototype.getZoomLevel = function (callback) {
+  if (typeof callback !== 'function') {
+    throw new Error('Must pass function as an argument')
+  }
+  process.nextTick(() => {
+    const zoomLevel = this._getZoomLevel()
+    callback(zoomLevel)
+  })
+}
+
+WebContents.prototype.getZoomFactor = function (callback) {
+  if (typeof callback !== 'function') {
+    throw new Error('Must pass function as an argument')
+  }
+  process.nextTick(() => {
+    const zoomFactor = this._getZoomFactor()
+    callback(zoomFactor)
+  })
+}
+
 // Add JavaScript wrappers for WebContents class.
 WebContents.prototype._init = function () {
   // The navigation controller.