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Also augments util::IdType to allow starting generators at different
values.

R=fdoray

Bug: 1136350
Change-Id: I48673d90e754371892f25021607d2986db8f0bce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461597
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816689}