[Memory Coordinator] Add the //base/ API

This change introduces the MemoryConsumer class and its associated
traits. It also adds the MemoryConsumerRegistry interface for
registering the consumers with a global instance.

This new class will be used as a replacement for
MemoryPressureListener, with additional capabilities and flexibility.

Bug: 406578344
Change-Id: I35b7afd2bb6742bc47894f8106c2085fc3d4dff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6648870
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1484687}