feat(puppeteer): introduce puppeteer.errors and puppeteer.devices (#4312)

These getters are introduced as a more convenient substitute for
a `require('puppeteer/Errors')` and
`require('puppeteer/DeviceDescriptors')`.

This way we can make cross-browser story nicer - a single require
of `puppeteer` or `puppeteer-firefox` fully defines Puppeteer
environment.

test/utils.js

@@ -70,13 +70,6 @@ const utils = module.exports = {
     return PROJECT_ROOT;
   },
 
-  /**
-   * @return {*}
-   */
-  requireRoot: function(name) {
-    return require(path.join(PROJECT_ROOT, name));
-  },
-
   /**
    * @param {!Page} page
    * @param {string} frameId