bookmarks: Add more fine-grained DCHECKs upon creation

IsValidIndex() captures the very same preconditions but calling out the
validity of the parent in dedicated DCHECKs helps readability and allows
understanding user reports better.

Bug: 1051614
Change-Id: If10e53fc52b3f6003dba9f9b2bf386ccf75de3e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2054165
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741114}

components/bookmarks/browser/bookmark_model.cc

@@ -610,6 +610,8 @@ const BookmarkNode* BookmarkModel::AddFolder(
     base::Optional<std::string> guid) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   DCHECK(loaded_);
+  DCHECK(parent);
+  DCHECK(parent->is_folder());
   DCHECK(!is_root_node(parent));
   DCHECK(IsValidIndex(parent, index, true));
 
@@ -640,6 +642,8 @@ const BookmarkNode* BookmarkModel::AddURL(
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   DCHECK(loaded_);
   DCHECK(url.is_valid());
+  DCHECK(parent);
+  DCHECK(parent->is_folder());
   DCHECK(!is_root_node(parent));
   DCHECK(IsValidIndex(parent, index, true));