feat: add WebContentsView since BrowserView is deprecated.

Add new WebContentsView class as BrowserView is deprecated.
Ref: electron#198

index.d.ts

@@ -27,6 +27,7 @@ export {
 export var app: Electron.App;
 export var autoUpdater: Electron.AutoUpdater;
 export var BrowserView: typeof Electron.BrowserView;
+export var WebContentsView: typeof Electron.WebContentsView;
 export var BrowserWindow: typeof Electron.BrowserWindow;
 export var clipboard: Electron.Clipboard;
 export var contentTracing: Electron.ContentTracing;