Introduce MemoryUsageMonitor

CrashMemoryMetricsReporterImpl, OomInterventionImpl, and the "additional critical memory pressure generator" we plan to implement, all require access to periodic memory usage data.

This CL introduces a MemoryUsageMonitor class that periodically checks the memory usage and notifies its observers.

Bug=920922

Change-Id: I1243ffa43369b6accedf3a0eb000230438c34883
Reviewed-on: https://chromium-review.googlesource.com/c/1404903
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625157}
8 files changed