The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/03fb66e4294e495210493648b496323ebf752abe

commit 03fb66e4294e495210493648b496323ebf752abe
Author: Scott Little <sclittle@chromium.org>
Date: Fri Jun 01 17:37:35 2018

LazyLoad: Log how often eligible frames are visible before loaded.

Record how often lazyload-eligible frames become visible before they're
finished loading for different effective connection types. This metric,
along with VisibleLoadTime
(https://chromium-review.googlesource.com/c/chromium/src/+/1069780),
should help with tuning the lazyload distance-from-viewport thresholds
at which a deferred frame is considered to be close enough to the
viewport to start loading it.

The VisibleLoadTime metrics aren't sufficient for tuning the lazyload
distance-from-viewport thresholds because on slow networks, iframes
might not finish loading by the time the user leaves the page, so the
slowest frames wouldn't be represented in the VisibleLoadTime samples.

Bug:  846932 
Change-Id: Ic0478c6cdc192c99250156b91938fc14735898ad
Reviewed-on: https://chromium-review.googlesource.com/1074384
Commit-Queue: Scott Little <sclittle@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563706}
[modify] https://crrev.com/03fb66e4294e495210493648b496323ebf752abe/third_party/blink/public/platform/web_effective_connection_type.h
[modify] https://crrev.com/03fb66e4294e495210493648b496323ebf752abe/third_party/blink/renderer/core/html/html_frame_owner_element.cc
[modify] https://crrev.com/03fb66e4294e495210493648b496323ebf752abe/third_party/blink/renderer/core/html/html_frame_owner_element_test.cc
[modify] https://crrev.com/03fb66e4294e495210493648b496323ebf752abe/tools/metrics/histograms/histograms.xml