Throttling: fix users stuck with invalid throttling settings

Following a CL last year (crrev.com/c/5582013) which landed in M127, we
did not correctly migrate users whose local settings may have become
invalid with that CL.

As a result of crrev.com/c/6317766, users with these old values end up
with a DevTools that crashes if they have emulation mode on, or if they
don't, an empty performance panel.

This CL implements two changes:

1. A migration to pull users with the old settings stored locally up to
   date with the new ones.
2. A fail-safe in the Network Condition serializer to ensure that if
   parsing goes wrong, we fallback to "No throttling". This is not
   ideal, but it is better than a broken DevTools.

As a future follow-up, we should update how we store these settings; in
particular the network conditions with `title` and `i18nPresetKey` is
confusing; we should have some stable, non-UI facing string/ID that we
use to look these up. Those IDs can then never change, and the user
strings can change safely without having to worry about persisted
settings.

R=szuend@chromium.org

Bug: 420384038
Change-Id: I805483d499da7eb762ab50378f9d008199823ba1
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6620929
Reviewed-by: Simon Zünd <szuend@chromium.org>
Auto-Submit: Jack Franklin <jacktfranklin@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>