Add HeuristicMemorySaverPolicy

This policy changes the memory saver algorithm to be based on a set of
experimental algorithms, the first iteration being a threshold of
available memory.

Bug: 1420344
Change-Id: I60c38b05d051fe923eb249a822a4ae25ce4724b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4327278
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Commit-Queue: Anthony Vallée-Dubois <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1117693}