Move files from chromecast_config to pref_service_helper.

1) Remove ChromecastConfig. Use PrefService directly.
2) Initialize PrefService after thread creation because it uses blocking IO
   browser thread instead of its own thread pool.
3) Use scoped_ptr for explicit ownership.

BUG=

Review URL: https://codereview.chromium.org/767503003

Cr-Commit-Position: refs/heads/master@{#307078}
22 files changed