This will be needed when retaining a back-buffer. Being able to
query the first index allows us to work out when we've discarded
all samples that were obtained from a particular chunk, which
we'll use to determine when to remove chunks from
ChunkSampleStream.mediaChunks.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175172085
