Actually, instead of changing the meaning of IsHigh, I will add "IsPreloaded" so it carries more information as we would be able to know the behaviour of preloaded origins more easily.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7623355495c159af6422997b8b52bcb299704d4c

commit 7623355495c159af6422997b8b52bcb299704d4c
Author: Mounir Lamouri <mlamouri@chromium.org>
Date: Tue Feb 27 16:56:30 2018

Media Engagement: add UKM metric to carry preloaded information.

Bug:  813905 
Change-Id: I7a0954620c62c7553397bdd13250f598589afb33
Reviewed-on: https://chromium-review.googlesource.com/928504
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539462}
[modify] https://crrev.com/7623355495c159af6422997b8b52bcb299704d4c/chrome/browser/media/media_engagement_session.cc
[modify] https://crrev.com/7623355495c159af6422997b8b52bcb299704d4c/tools/metrics/ukm/ukm.xml