[SelfShare] Extend GCMMessage object to access original_priority value.

Adds a method to get the original_priority of the message. Also adds
the code needed to (de)serialize as well as some unit tests.

Bug: 939840
Change-Id: I9dfd3d7b203857205f774eabfa9b084ad932bf6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1511695
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639917}
2 files changed