[Chromecast] Raises CastService creation to CastContentBrowserClient.

This is a first step toward resolving the chromecast/service/ --> content/
dependency by removing the link-time creation path in CastService.

Also, removes some code that has become dead via refactoring across the
internal/external repository boundary.

R=slan@chromium.org,halliwell@chromium.org,byungchul@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/1327723002

Cr-Commit-Position: refs/heads/master@{#346936}
15 files changed