Chromecast: makes CastWindowAndroid use CastContentWindow.

This unifies the WebContents creation code between each platform.

R=lcwu@chromium.org,byungchul@chromium.org
BUG=internal b/18934906

Review URL: https://codereview.chromium.org/874683006

Cr-Commit-Position: refs/heads/master@{#315818}