Reland "SyncToSigninMigration: Implement SyncPrefs migration"

This relands commit 7d3f6d0c39c8dcf71ac1c8b1192debf67b414a1d
(and reverts commit 4b47094cfe185c794671b3d820ea47221723466e).

Reason for reland: The original CL was reverted due to a conflict
with crrev.com/c/4874142 (which landed almost simultaneously).
PS1 is the original CL.
PS3 contains the fixes: One desktop-specific addition to the
migration logic, and some minor test adjustments.

Original change's description:
> [Gardening] Revert "SyncToSigninMigration: Implement SyncPrefs migration"
>
> This reverts commit 7d3f6d0c39c8dcf71ac1c8b1192debf67b414a1d.
>
> Reason for revert: Causing failures on Mac.
>
> Original change's description:
> > SyncToSigninMigration: Implement SyncPrefs migration
> >
> > This implements the migration of the "global" data-type-selected prefs
> > (used for syncing users) to the per-account prefs (used for signed-in
> > non-syncing users).
> >
> > Bug: 1486420
> > Change-Id: I3d6faced9efa9a1b3ad4813b28f5b3f16960856d
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5033598
> > Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
> > Reviewed-by: Victor Vianna <victorvianna@google.com>
> > Commit-Queue: Marc Treib <treib@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#1228414}
>
> Bug: 1486420
> Change-Id: I7b7b5894dcaf617e65e827e48e8686f56a1617bd
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5054404
> Commit-Queue: Ian Vollick <vollick@chromium.org>
> Reviewed-by: Marc Treib <treib@chromium.org>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1228455}

Bug: 1486420
Change-Id: I88b23c209048bd274d5ccc9bf7dbf06848a16e18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5053896
Reviewed-by: Victor Vianna <victorvianna@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1228757}