Add a MetricEvaluator class to SystemMonitor

This CL is a response to this comment: https://chromium-review.googlesource.com/c/chromium/src/+/1349670/15/chrome/browser/performance_monitor/system_monitor.h#12

The goal is to make it easier to add a new metric to SystemMonitor and
to reduce the risk of errors by reducing the number of functions that
have to be updated when doing so.

A follow up CL will template this class: https://chromium-review.googlesource.com/c/chromium/src/+/1474810
(this is done separately to reduce the complexity).

Change-Id: I97129d6db9897af1d6e0283e5d37bea28c77ed26
Reviewed-on: https://chromium-review.googlesource.com/c/1466189
Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634284}
13 files changed