[iOS][Bkmrk] GetDefaultBookmarkFolder() should only return a folder

This patch adds a DCHECK() when setting the default folder, to be sure
the BookmarkNode is a folder.
And this patch adds a test when getting the default folder from
the pref, to not return a BookmarkNode that is not a folder.
We should not trust pref values.

This patch needs to be cherry-picked in M115 and M114 if possible.

(cherry picked from commit 4c37398)

Fixed: 1450146
Change-Id: I847a30c7d57d0685843e090e56b8911065bdbdf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4577200
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1151686}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4593716
Reviewed-by: Menghan Yang <myuu@google.com>
Commit-Queue: Menghan Yang <myuu@google.com>
Cr-Commit-Position: refs/branch-heads/5790@{#402}
Cr-Branched-From: 1d71a33-refs/heads/main@{#1148114}

ios/chrome/browser/bookmarks/bookmarks_utils.cc

@@ -86,6 +86,8 @@ void SetLastUsedBookmarkFolder(PrefService* prefs,
                                const bookmarks::BookmarkNode* folder,
                                bookmarks::StorageType type) {
   DCHECK(folder);
+  DCHECK(folder->is_folder()) << "node type: " << folder->type()
+                              << ", storage type: " << static_cast<int>(type);
   prefs->SetInt64(prefs::kIosBookmarkLastUsedFolderReceivingBookmarks,
                   folder->id());
   prefs->SetInteger(prefs::kIosBookmarkLastUsedStorageReceivingBookmarks,
@@ -108,7 +110,7 @@ const bookmarks::BookmarkNode* GetDefaultBookmarkFolder(
         type, profile_bookmark_model, account_bookmark_model);
     const BookmarkNode* result =
         bookmarks::GetBookmarkNodeByID(bookmark_model, node_id);
-    if (result) {
+    if (result && result->is_folder()) {
       return result;
     }
   }