Add StreamModelUpdateRequest and functions for converting wire protos into store protos

Bug: 1044139
Change-Id: I243ebabc331884500dec480c69d843d38546a34e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2020545
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747983}