This also adds support for seeking in periods with midroll ads.
Remove Timeline.Period.isAd.
Issue: #2617
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160510702
This will allow MediaSources to provide MediaPeriods that correspond to ad
breaks in a timeline period rather than content for a timeline period, in a
future change.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160267841
continueLoading may be called during preparation, but this is not
handled correctly in the case where a deferred period doesn't
have a source yet.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158696539