Move logic for site details histograms into MetricsMemoryDetails.

This CL is a refactor with no intended behavior change.

This CL is the beginning of a refactor to separate the two types of consumers of
MemoryDetails:
  * Those that want a human readable string that shows a mapping from [process :
  memory usage].
  * Those that want to emit non-memory related metrics.

Bug: 864352
Change-Id: I0f2a968ef63b3c9fcb658898a2f3f55f68867285
Reviewed-on: https://chromium-review.googlesource.com/1170587
Reviewed-by: Nick Carter <nick@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582243}