The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25

commit 4ad49e0922bc1efd2e377aa06e4cacceff6d0d25
Author: Scott Little <sclittle@chromium.org>
Date: Wed May 30 18:24:06 2018

LazyLoad: Record visible load time for eligible cross-origin iframes.

This CL adds support for recording visible load time for
lazyload-eligible iframes, which is the delay between a frame becoming
visible and that frame finishing loading. Visible load time is recorded
separately for above-the-fold and below-the-fold frames, and isn't
recorded for hidden (e.g. tiny or offscreen above or to the left of the
page) frames.

Since visible load time relies on intersection observers, and creating
lots of intersection observers could potentially harm performance (e.g.
 https://crbug.com/817250 ), the measuring and recording of this feature
is behind a flag.

Bug:  827717 
Change-Id: I85e8ac8d0f426b0dbac4262147e5d1d556a00d41
Reviewed-on: https://chromium-review.googlesource.com/1069780
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562918}
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/content/child/runtime_features.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/content/public/common/content_features.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/content/public/common/content_features.h
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/public/platform/web_runtime_features.h
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/html/html_frame_owner_element.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/html/html_frame_owner_element.h
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/html/html_frame_owner_element_test.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/testing/sim/sim_test.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/testing/sim/sim_test.h
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/testing/sim/sim_web_frame_client.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/core/testing/sim/sim_web_frame_client.h
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/platform/exported/web_runtime_features.cc
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/third_party/blink/renderer/platform/runtime_enabled_features.json5
[modify] https://crrev.com/4ad49e0922bc1efd2e377aa06e4cacceff6d0d25/tools/metrics/histograms/histograms.xml