[Bookmarks Bar] Fix missing no bookmarks empty state on sub menu folders

This CL fixes a bug in the bookmarks bar for the empty state of sub menu
folders. We currently look at the count of the number of items to
determine the minimum height and whether or not to show the no bookmarks
empty state. However, for sub menus we get a non-zero value because they
have a header. This CL updates to look at the content adapter for the
total count.

Bug: 445752498
Change-Id: Ib31d464fd7baf776a744e498dcdf947423685784
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6967617
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Jenna Himawan <jhimawan@google.com>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518721}