Adding a memory pressure observer for ChromeOS.

This is a memory observer for ChromeOS which triggers the existing memory pressure listening objects to release memory.

Part of this code came from the old Athena project (see https://codereview.chromium.org/5113830031).

Note that all this code resides for the time being behind a flag.

BUG=439493
TEST=-

Review URL: https://codereview.chromium.org/788713003

Cr-Commit-Position: refs/heads/master@{#307759}
11 files changed