[RSG] Refactor pending queries in AutocompleteHistoryManager

Given that a query can only be started in
AHM::OnGetSingleFieldSuggestions() and the fact that before starting
another query, AHM::OnGetSingleFieldSuggestions() cancels the previous
one, there can only be (at most) one pending query at a given time.
Because of this, std::map is not needed and a simple struct can be used
to store the pending query.

Bug: 409962888
Change-Id: Iab48c30844df9188ce292447bc36c0ebfb23da82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6708447
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Karol Sygiet <sygiet@google.com>
Cr-Commit-Position: refs/heads/main@{#1483678}