When buffering playlist items after the currently-playing one,
continueBuffering should take a position that is negative because the playback
position is before the start of the source being buffered. This change makes
sure that ExtractorSampleSources always have a known duration when they are
fully buffered, which means that the correct (negative) source-relative
position can be calculated.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124949409
