Add FlatBuffer Migrate Stale Tabs to about_flags

This enables us to test the two versions of the experiment on canary:
1) New and existing Tabs which are interacted with are migrated
to the new TabState FlatBuffer format
2) 1) + migrate stale Tabs as well. In addition to 1), Tabs which
have gone un-interacted with are migrated to the new TabState
FlatBuffer file format in batches of 5.

Bug: b:334854848, b:334620540
Change-Id: I85d60e4d130f98087ee9b72dce6acaae7d1c5816
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5516510


Commit-Queue: David Maunder <davidjm@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300858}