Chromecast: adds CastNetworkDelegate.

This provides an interface for special handling of "whitelisted" URLs
in internal code.

It also denies local file access for Chromecast, except on Android
(with a flag).

R=lcwu@chromium.org,byungchul@chromium.org
BUG=336640

Review URL: https://codereview.chromium.org/598303002

Cr-Commit-Position: refs/heads/master@{#297079}
12 files changed