This allows us to use one underlying buffer rather than 3, and allows memory to
be freed as we're growing the vector.

Change-Id: I45c178a31e8f6d3ee44d3319ce8bca2db2460d33
Reviewed-on: https://chromium-review.googlesource.com/c/1297328Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56966}