Migrate DictionaryPrefUpdate usage in update code.

DictionaryPrefUpdate is deprecated. ScopedDictPrefUpdate only allows
access via the newer Value::Dict API, and returns references rather
than pointers (The pref system itself has CHECKs/DCHECKs that the
value is registered and of the right type, so consumers do not need to
check that).

Also fix some consumers that shouldn't be using ScopedDictPrefUpdate.
Consumers that only read the value should not use it, to avoid sending
a bogus pref update notification, and consumers that completely
overwrite the value should use be using PrefService::SetDict(), to
reduce code size (and potentially slightly improve perf).

Bug: 1362719
Change-Id: Ic6044adc361215cc8a7f94a8e5ed1e8c4d195f73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3915492
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1051210}

• chrome/updater/persisted_data.cc[diff]
• chrome/updater/persisted_data.h[diff]
• components/update_client/persisted_data.cc[diff]
• components/update_client/persisted_data.h[diff]