Replace first_run_helper.mojom by C++ interface

TBR=tsepez@chromium.org

Bug: 958200
Test: trybot
Change-Id: I28cfc044779d72c5be48ad74639b5b5fcc10bb9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618544
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661848}
20 files changed