security: allow to block desktopCapturer.getSources() calls (#15964)

* security: allow to block desktopCapturer.getSources() calls

* return empty instead of error

* fix: release resources of DesktopCapturer on exit

lib/browser/api/web-contents.js

@@ -377,6 +377,10 @@ WebContents.prototype._init = function () {
     })
   })
 
+  this.on('desktop-capturer-get-sources', (event, ...args) => {
+    app.emit('desktop-capturer-get-sources', event, this, ...args)
+  })
+
   this.on('remote-require', (event, ...args) => {
     app.emit('remote-require', event, this, ...args)
   })