This adds two options to the ClippingMediaSource which allow proper clipping
of live streams:
1. The clipping stays fixed relative to already created media periods. That
means that playback actually progresses through the clipped media and
eventually reaches the end of the clipping. The window is also marked
as non-dynamic to let playback end in this case.
2. Allow to specify a clipping duration relative to the default position to
be able to specify the duration of live stream which is to be played.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190911049
39e4112e8f