[omnibox][webui] Cleanup `SearchboxHandler` `Create*()` methods

- Split `SearchboxHandler::CreateAutocompleteMatches()` into
  `CreateAutocompleteMatches()` and `CreateAutocompleteMatch()`.
- Reorder some methods (Result > SuggestionGroup > Matches > Match)
- Mark some params and methods `const`
- Remove `const` from some methods that were returning non-const owned
  pointers

This is in preparation for crrev.com/c/6995285 . No behavior change.

Bug: 445510209, 445456498
Change-Id: Ie825759353cca29ed5412fbb581f41f01725d12f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7001516
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523661}