Chromecast: makes CastContentWindow a WebContentsObserver.

This allows for common WebContentsObserver functionality to live within
CastContentWindow instead of in WebApplication.

R=lcwu@chromium.org,derekjchow@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/826933006

Cr-Commit-Position: refs/heads/master@{#310315}
2 files changed