Split out of https://webrtc-review.googlesource.com/c/src/+/165389.
I disentangled the plottable counter printer from the perf result
printer so it will work for both future implementations of the perf
test JSON writers. The only thing plottable counters and the
results writer had in common was that both wrote JSON anyway.
Bug: chromium:1029452
Change-Id: I041c3096641eda42542e8d994b246eb313940b4b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165397
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30198}
e1cbb9c20e