See the public design note https://docs.google.com/document/d/1r4HfiRHfhkPx9LOVkK0QAdq6H9coIxGH0nh6HojDnYE/edit?usp=sharing for the spec of these field trial params.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/878f7d6706331673d5adfeeb992c0a3e33b83c9c

commit 878f7d6706331673d5adfeeb992c0a3e33b83c9c
Author: Scott Little <sclittle@chromium.org>
Date: Thu May 31 22:09:06 2018

LazyLoad: Make lazy frame loading distance threshold configurable.

This CL makes the distance-from-viewport threshold that controls how
close the user has to scroll to a deferred iframe configurable via
--blink-settings and field trial params. Different distance threshold
values can be specified for different effective connection types and
data saver settings, so that e.g. a larger distance threshold can be
specified for slower networks to give frames more time to load in, or
e.g. a shorter distance threshold could be specified for data saver
users to save some more data.

The spec for these field trial parameters is here:
https://docs.google.com/document/d/1r4HfiRHfhkPx9LOVkK0QAdq6H9coIxGH0nh6HojDnYE/edit#

Bug:  845307 
Change-Id: I9922b7d5dc863c8e62bd9a496c87a31c08f544e2
Reviewed-on: https://chromium-review.googlesource.com/1069338
Commit-Queue: Scott Little <sclittle@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563393}
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/content/public/common/common_param_traits_macros.h
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/content/public/common/web_preferences.h
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/content/renderer/render_view_impl.cc
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/public/web/web_settings.h
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/renderer/core/exported/web_settings_impl.cc
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/renderer/core/exported/web_settings_impl.h
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/renderer/core/frame/settings.json5
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/renderer/core/html/html_frame_owner_element.cc
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/renderer/core/html/html_frame_owner_element.h
[modify] https://crrev.com/878f7d6706331673d5adfeeb992c0a3e33b83c9c/third_party/blink/renderer/core/html/html_frame_owner_element_test.cc