Circular dependency between offline.Storage and offline.DownloadManager

[joeyparrish]

In #421, Andy is trying to upgrade us to a newer compiler. That compiler caught this circular dependency as a missing goog.require.

offline.Storage requires offline.DownloadManager, but DownloadManager has this:

  var manifest = shaka.offline.Storage.getStoredContent(this.manifest_);
  this.config_.progressCallback(manifest, progress);

We should clean up this cycle.