The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d07a4a173711822c89059dea4b3557964583976a

commit d07a4a173711822c89059dea4b3557964583976a
Author: Eugene But <eugenebut@google.com>
Date: Tue Aug 21 14:53:15 2018

Add NavigationContextImpl::IsLoadingHtml.

This boolean will be true for loadHTMLString: navigations.

-[CRWWebController setDocumentURL:] will have a DCHECK when new document
URL does not match last committed URL in WKBasedNavigationManager. This
DCHECK will help to diagnose possible URL spoofing bugs in Slim
Navigation feature.

For loadHTMLString: the URLs will not match, which is not a problem
because loadHTMLString: are only used in tests and error pages.

Bug:  876077 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Iae163faf7f5cf90d5bb0e48a191786be2e1d8ee6
Reviewed-on: https://chromium-review.googlesource.com/1182313
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584740}
[modify] https://crrev.com/d07a4a173711822c89059dea4b3557964583976a/ios/web/web_state/navigation_context_impl.h
[modify] https://crrev.com/d07a4a173711822c89059dea4b3557964583976a/ios/web/web_state/navigation_context_impl.mm
[modify] https://crrev.com/d07a4a173711822c89059dea4b3557964583976a/ios/web/web_state/ui/crw_web_controller.mm

UKM collection doc:
https://docs.google.com/document/d/14SygQ8BzLkzedTLuDfOHhakQHZOOJSU8TAJT3rxBGv4/edit#