[omnibox][ios] Provide autocompletion for account bookmarks

This patch extends BookmarkProvider to support up to two underlying
BookmarkModel instances: one for the regular (so-called
local-or-syncable) instance (bookmarks::StorageType::kLocalOrSyncable),
and the second one (behind feature toggle) representing account
bookmarks (bookmarks::StorageType::kAccount).

The implementation queries both models independently and merges results
afterwards. The ranking of matches is generally independent, except for
the boosting of scores based on bookmarked URLs, which is
model-agnostic (i.e. one model can influence the matches returned by
the other).

Outside ios, the second BookmarkModel instance is null, as becomes
obvious in +ChromeAutocompleteProviderClient::GetAccountBookmarkModel().

Change-Id: If3afd819058d2611f9cc67d1f0f6ff31f92102f0
Bug: 1424825
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4547697


Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146408}