[bookmarks] Refactor APIs to introduce enum NodeTypeForUuidLookup

No behavioral changes, as the enum initially contains a single value.

The general idea is that bookmark UUIDs are guaranteed to be unique only
in the scope of each NodeTypeForUuidLookup value. That's because
duplicate bookmarks could co-exist in certain circumnstances within a
single BookmarkModel instance, which also means they share UUIDs.

These duplicate UUIDs cannot exist in arbitrary locations of the
bookmark tree. Instead, NodeTypeForUuidLookup represents how the tree
can be partitioned in subtrees, where uniqueness is guaranteed for each
of the substrees (enum values).

In this first patch, a single enum value is introduced to keep the
delta small and avoid behavioral changes. Follow-up patches will extend
the enum with a second value that represents account bookmarks.

Design doc (sorry, Googlers only):
https://docs.google.com/document/d/1i9XuP0lXOvEbvlwXre-EXMum4dxs_UToCsCcImIPEQU/edit?usp=sharing&resourcekey=0-1t7DIB0kTw\
aaGMmxtiZLqw

Change-Id: I6d9c98fb8f7f6b1b51974f8555c4aedcf1714f92
Bug: 1494120
Low-Coverage-Reason: TRIVIAL_CHANGE concerning chrome/browser/
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5189787
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1247776}