[First Run Bookmark Import] Add InitialPreferences::GetBookmarksBlock

This CL adds the GetBookmarksBlock method to load Bookmarks Block from
Initial Preferences file upon first run.

Fixed: 433262898
Change-Id: I1f5c7727584a4c899b45599668512bb777697f9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6793224
Reviewed-by: Kaan Alsan <alsan@chromium.org>
Commit-Queue: Atharv Maan <atharvmaan@chromium.org>
Reviewed-by: Daniel Soromou <koretadaniel@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1497295}