rule-of-2 - add protobuf shared memory note and ProtoWrapper

Change-Id: Ic49a4eb0ea3a5a2d07b2e071d08ca112ff1778b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406234


Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279968}