[ios] Update HistoryService to deal with a unified BookmarkModel

Behavioral changes only behind disabled feature flag.

When `syncer::kEnableBookmarkFoldersForAccountStorage` is enabled, there
is only one BookmarkModel instance per BrowserState and hence
HistoryClient cannot and should not expose two.

The effect of enabling the flag is similar to reverting
https://crrev.com/c/4531967.

Bug: 326185948
Change-Id: I7463fb89fe3a73356fdbafa758e71af6c983c3ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5352967


Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270116}