[MemoryCache] Implement base::MemoryConsumer

By implementing this interface, MemoryCache limit its memory usage when
signaled by the memory coordinator.

This will be used in an experiment to clear strong references during a
last resort GC.

Note: While MemoryConsumer is eventually meant to replace
MemoryPressureListener, it currently does not have overlapping
functionality.

Bug: 447352454
Change-Id: If17e42d4158643e6105bf4af9b384ba04832f1c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6986421
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525705}