Extend histogram expiration milestone

Bug: 1138316, 1138317
Change-Id: Ib2ce5f4114643e3e463f413bd01cd0934d00c3ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512897


Auto-Submit: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823261}