[iOS][Bookmarks] Fix BookmarkModel::IsBookmarked usages in UI code

Prior to this CL, some iOS-specific UI code paths related to overflow
menu and tab grid were calling BookmarkModel::IsBookmarked and
GetMostRecentlyAddedUserNodeForURL directly and only checked
LocalOrSyncable model. This CL addresses that by introducing
bookmark_utils_ios::IsBookmarked that checks both model.

This CL also changes the behavior of
GetMostRecentlyAddedUserNodeForURL, which now allows passing null for
LocalOrSyncable model (this is done for consistency with
IsBookmarked).

Bug: 1447996
Change-Id: I53d14d2d23ee50be787762f8d54cf4ea7e9bd8c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4553840


Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148099}