Update reopen tab in-product help histogram expiry

We are tentatively aiming for a M75 full stable launch. Bumping the
expiry to M76 will allow us to still get data from Canary/Dev after
M75 branch.

Bug: 945576, 887991
Change-Id: I35cd8c4d55a20bf724a5e532607c8d3f4a83ea47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539136
Auto-Submit: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644963}