ProgressiveDownloader never implemented this properly, and we
don't use it in DownloadManager/DownloadService, both of which
use a polling model. A polling model is also what's used elsewhere,
for example to query the current playback position.
This is effectively doing the TODO in ProgressiveDownloader.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194214579
