[BrowserMemoryConsumerRegistry] Sever child registry connections in dtor

The normal lifetime of those connections is that they are cleaned up in
OnChildMemoryConsumerDisconnected. During shutdown, it's better to skip
waiting for those disconnection notifications and proactively sever
them.

This is necessary in browser tests where the
BrowserMemoryConsumerRegistry is destroyed before all child registries
are properly cleaned up.

Bug: 447352454
Change-Id: I5789448e51b691b39f90be0fedb600b9fd629b80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6995759
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522390}