This is in preparation for the player knowing about the live
window, at which point only correct playback positions will be
passed into ChunkSource implementations. Note that both
implementations bound the chunk index within range in case of,
for example, off-by-one errors caused by very recent manifest
updates.
This will temporarily cause live playbacks to always start at
the beginning of the current live window (we'll be trying to
play from t=0, which will be adjusted to the start of the live
window as a result of the bounding).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119733559
f963c626af