static async supported() {
    // We need our own ID for Karma to look up the WebDriver connection.
    // For manually-connected browsers, this ID may not exist.  In those cases,
    // this method is expected to return false.
    const urlParams = location.search;

    const buffer = await shaka.test.Util.fetch(
        '/screenshot/isSupported' + urlParams);
    const json = shaka.util.StringUtils.fromUTF8(buffer);
    const ok = /** @type {boolean} */(JSON.parse(json));
    return ok;
  }
Checks with Karma to see if this browser can take a screenshot.

Only WebDriver-connected browsers can take a screenshot, and only Karma
knows if the browser is connected via WebDriver.  So this must be checked
in Karma via an HTTP request.

@return {!Promise<boolean>}