eb4e05199f
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