addMonitor(threshold, monitorPtr, callback) {
    return this.handler.addMonitor(threshold, monitorPtr);
  }
This is a testing framework that enables us to test the user idle detection
by intercepting the connection between the renderer and the browser and
exposing a mocking API for tests.

Usage:

1) Include <script src="mock.js"></script> in your file.
2) Set expectations
  expect(addMonitor).andReturn((threshold, monitorPtr, callback) => {
    // mock behavior
  })
3) Call navigator.idle.query()

The mocking API is blink agnostic and is designed such that other engines
could implement it too. Here are the symbols that are exposed to tests:

- function addMonitor(): the main/only function that can be mocked.
- function expect(): the main/only function that enables us to mock it.
- function close(): disconnects the interceptor.
- enum UserIdleState {IDLE, ACTIVE}: blink agnostic constants.
- enum ScreenIdleState {LOCKED, UNLOCKED}: blink agnostic constants.