Note: for *PaintTiming* there are two observers currently for ClientLoFi of OfflinePage client-side previews: 

chrome/browser/page_load_metrics/observers/previews_page_load_metrics_observer.cc chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc

so need to decide whether to add to one of these.

And for *Experimental.Bytes.Network for ClientLoFi, there is 
chrome/browser/page_load_metrics/observers/lofi_page_load_metrics_observer.cc
(that doesn't handle PaintTiming).

Should we just add noscript_page_load_metrics_observer to handle both PaintTiming and Experimental.Bytes.External?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bb0049744f868042dc3ceaf1edfdb3b8c73271a1

commit bb0049744f868042dc3ceaf1edfdb3b8c73271a1
Author: Doug Arnett <dougarnett@chromium.org>
Date: Mon Nov 27 17:54:11 2017

Adds timing and data UMA variants for NoScript previews

Bug:  781892 
Change-Id: I915dc2beb47a5f879eab955ad8d4377e8c0747ea
Reviewed-on: https://chromium-review.googlesource.com/772812
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519334}
[modify] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/BUILD.gn
[add] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/observers/noscript_preview_page_load_metrics_observer.cc
[add] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/observers/noscript_preview_page_load_metrics_observer.h
[add] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/observers/noscript_preview_page_load_metrics_observer_unittest.cc
[rename] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer.cc
[rename] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer.h
[rename] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer_unittest.cc
[modify] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/browser/page_load_metrics/page_load_metrics_initialize.cc
[modify] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/chrome/test/BUILD.gn
[modify] https://crrev.com/bb0049744f868042dc3ceaf1edfdb3b8c73271a1/tools/metrics/histograms/histograms.xml