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This metric tracks how long the user has to wait for a visible image to
load completely. This metric will help understand how much delay does
lazyload of images incurs users.

Bug: 812684
Change-Id: Ic97c9a192254a2b64d9f2427d8694d6e36759db9
Reviewed-on: https://chromium-review.googlesource.com/831530
Commit-Queue: rajendrant <rajendrant@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Scott Little <sclittle@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#539221}