Keeping a persistent storage allows us to treat the memory
buffer as the canonical copy of the pack buffer data, and
simplifies corner cases.

BUG=angle:511

Change-Id: Ied4dcb2edce2b85f68645f5a65603cbf18ad5a16
Reviewed-on: https://chromium-review.googlesource.com/197701Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>