[Chromecast] Lowers CastService out of chromecast/browser/.

This creates a chromecast/service/ directory with no dependencies on content/.

R=slan@chromium.org,halliwell@chromium.org
BUG=internal b/23761158

Review URL: https://codereview.chromium.org/1319273003

Cr-Commit-Position: refs/heads/master@{#347739}
14 files changed