refactor: move ChromiumDownloader under lib/ (#1554)

This patch:
- renames ChromiumDownloader into just Downloader (this is in
  preparation for different products download)
- moves Downloader from utils/ to lib/. This unifies all of the
  production-critical code in the lib/.

Drive-by: make Downloader a regular class.

utils/doclint/check_public_api/index.js

@@ -21,6 +21,7 @@ const Message = require('../Message');
 
 const EXCLUDE_CLASSES = new Set([
   'Connection',
+  'Downloader',
   'EmulationManager',
   'FrameManager',
   'Helper',