Bundled version of Puppeteer should rely on native WebSocket.
Luckily, 'ws' module supports the same interface as the native
browser websockets. This patch switches WebSocketTransport to
use the browser-compliant interface of 'ws'.
After this patch, I was able to bundle Puppeteer for browser
using the following config in `package.json`:
```json
"browser": {
"./lib/BrowserFetcher.js": false,
"ws": "./lib/BrowserWebSocket",
"fs": false,
"child_process": false,
"rimraf": false,
"readline": false
}
```
where `./lib/BrowserWebSocket` is:
```js
module.exports = WebSocket;
```
and the bundling command is:
```sh
$ browserify -r ./index.js:puppeteer > ppweb.js
```
References #2119
Currently connection assumes that transport is a websocket
and tries to handle websocket-related errors.
This patch:
- moves ConnectionTransport interface to use callbacks instead
of events. This way it could be used in browser context as well.
- introduces WebSocketTransport that implements ConnectionTransport
interface for ws.
This is a preparation step for 2 things:
- exposing `transport` option in the `puppeteer.connect` method
- better support for `browserify`
References #2119
