Don't offer profile imports for obvious corrections

If the user autofills a profile and edits a single autofilled value
before submission, new profile creation is currently offered.
With this CL, updating the existing profile with the newly observed
value is offered instead, if the type of the edited field has low
quality in the existing profile.

This is done by:
- Augmenting ProfileImportMetadata with a map
  (FieldType -> std::optional<std::string>). The map contains an entry
  for every observed field type that was used to construct the
  candidate profile. Nullopt indicates that the field wasn't autofilled.
  Otherwise, the string value is the GUID of the profile that was used
  to autofill this field.
  As a result, the Chromium style checker considers
  ProfileImportMetadata a "complex struct", requiring an user-defined
  constructor. Thus, aggregate initialisation no longer works and
  requires updating a few uses of the struct around the codebase.
- Adding logic to ProfileImportProcess::DetermineProfileImportType() to
  detect the case mentioned above and change the import type and
  merge candidate accordingly.

Bug: b/325451601
Change-Id: Iaaea098074b882de8b329bb1dd41f4e815d121f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412616
Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1301313}