Fix dangling ptrs in UpdateApplicationDockMenu test

UpdateApplicationDockMenu was previously creating Browser obejcts,
passing pointers to BrowserWindow unique_ptrs that were destroyed
sortly after. The result was Browser objects with dangling
BrowserWindow pointers.

This CL corrects this by ensuring BrowserWindow unique_ptrs are
persisted and destroyed only after their associated Browser has
been destroyed.

Bug: 417766643
Change-Id: I968c773971cd4722abc987e60baefaad5a46c70e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6605059
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Auto-Submit: Tom Lukaszewicz <tluk@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1467519}