SDK / exoplayer

The mediaChunks.size() > 1 check was supposed to ensure this, and
did roughly the right thing when there was only a single stream
(although it was unnecessarily restrictive in preventing chunk
cancelation for the first chunk, where bytesLoaded != 0 and none
of the samples had been consumed).

Now we have multiple streams the check doesn't do the right thing,
and adding a back-buffer feature will make even more incorrect.
This change switches to checking the condition we actually want
to check directly :).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=174449398