Push expiry on thumbnail capture histograms.

We're still working on this feature, so we don't want to expire them.

Bug: 984201
Change-Id: Ibdd0b6c9f77bc02779828a1c5f8108e568e746e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719146
Auto-Submit: Dana Fried <dfried@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681109}