GitLab

- make CastSocketService a global leaky singleton and remove CastSocketServiceFactory
- make CastChannelAPI and CastMediaSinkService own CastSocket::Observer
- fix unit tests and browser tests

BUG=687377

Review-Url: https://codereview.chromium.org/2974523002
Cr-Commit-Position: refs/heads/master@{#486886}