Maintain bookmark order across synced devices

[ayumi]

Test plan:

1. Pyramid 0: Enable Sync and make some bookmarks.
2. Pyramid 1: Enable Sync and add to existing sync profile.
3. Pyramid 1: Bookmarks bar looks like {Pyramid 1 bookmarks}{Pyramid 0 bookmarks}.
4. Pyramid 0: Bookmarks bar looks like {Pyramid 0 bookmarks}{Pyramid 1 bookmarks} – out of sync.

Expected behavior:

• Bookmark order should be preserved across devices.
• For example, assume existing sync records take precedence. Therefore in Step 3, Pyramid 1 bookmarks should look like {Pyramid 0 bookmarks}{Pyramid 1 bookmarks} (pyramid 0 bookmarks prepended).