Simplify SingleFieldFormFillRouter

SingleFieldFormFillRouter currently holds WeakPtrs to the
AutocompleteHistoryManager, IbanManager and MerchantPromoCodeManager.

This is unnecessary, given the lifetimes of the four classes:
- AutocompleteHistoryManager, IbanManager and MerchantPromoCodeManager
  are KeyedServices.
- SingleFieldFormFillRouter is owned by the BAM, which is per frame.

Even after we refactor the BAM to be per tab, the KeyedServices will
always outlive SingleFieldFormFillRouter.

As such, this CL:
- Removes the WeakPtrFactories from the AutocompleteHistoryManager,
  IbanManager and MerchantPromoCodeManager.
- Makes the members in SingleFieldFormFillRouter raw_refs/raw_ptrs (the
  AutocompleteHistoryManager always exists, but the other two are not
  supported on some platforms).
- Removes an unnecessary null check.
- Cleans up the SingleFieldFormFillRouter tests a bit:
  - Instead of having pointers to all relevant classes in the fixture,
    just store the values.
  - Reorder the members, so the destruction order matches reality.

Bug: 1007974
Change-Id: I0e379f3ca93648ec3b9f034d000d4a5e6fcb8637
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5071534
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1240199}