Content progress is only polled if there are midroll ad groups. If the ad tag
had only preroll/postroll ads, the pending content position was not provided to
IMA, which meant that the postroll ad could never play.
Only set the pending content position if there are midroll ads.
Issue: #3715
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185680803
