Add "enable_desktop_capturer" build flag (#13133)

* Make it possible to disable a module for a renderer

* Put DesktopCapturer API under a build flag

The name is "enable_desktop_capturer".
Enabled by default.

lib/sandboxed_renderer/api/exports/electron.js

@@ -33,6 +33,8 @@ Object.defineProperties(exports, {
       return require('../../../common/api/is-promise')
     }
   },
+  // XXX(alexeykuzmin): It won't be available if the Desktop Capturer
+  // was disabled during build time.
   desktopCapturer: {
     get: function () {
       return require('../../../renderer/api/desktop-capturer')