[Memory Coordinator] Instantiate browser and child registries

This CL adds the instantiation of the BrowserMemoryConsumerRegistry
in the browser process, and the ChildMemoryConsumerRegistry in all
child processes, enabling the registration of base::MemoryConsumer
instances in all processes.

Bug: 406578344
Change-Id: Ib100449309ef17f49fc84f2f7706200ea36e8238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6701316
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1490803}