function frame() {
    var frameId = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : null;

    return this.requestHandler.create('/session/:sessionId/frame', { id: frameId });
}
Change focus to another frame on the page. If the frame id is null,
the server should switch to the page's default content.

<example>
    :frame.js
    it('should switch focus to iFrame', function () {
        // Using `element` to find an iframe and providing it to `frame` method
        browser.waitForExist('iframe[name="my_iframe"]');
        var my_frame = $('iframe[name="my_iframe"]').value;
        browser.frame(my_frame);
    });
</example>

@param {String|Number|null|WebElementJSONObject} id   Identifier for the frame to change focus to.

@see  https://w3c.github.io/webdriver/webdriver-spec.html#switch-to-frame
@type protocol