We currently switch without downloading overlapping segments, but
we do not actually switch more aggressively. This change fixes
this. Note there's an implicit assumption made that if one media
playlist declares independent segments, the others will too. This
is almost certainly true in practice, and if it's not the penalty
isn't too bad (the player may try and switch to a higher quality
variant one segment's worth of buffer too soon).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167120992
d942775068