refactor: move Connection to use ConnectionTransport (#3217)

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

utils/doclint/check_public_api/index.js

@@ -31,9 +31,10 @@ const EXCLUDE_CLASSES = new Set([
   'Multimap',
   'NavigatorWatcher',
   'NetworkManager',
-  'Pipe',
+  'PipeTransport',
   'TaskQueue',
   'WaitTask',
+  'WebSocketTransport',
 ]);
 
 const EXCLUDE_PROPERTIES = new Set([