An alternative would be to log Content-Length (or just a boolean, WasContentLengthZero) in UKM.

See also  issue 808205  which seems to say that empty responses need to be "blocked".

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/80693a3b5a96e9e390bafb401cd46a0feb0a61fd

commit 80693a3b5a96e9e390bafb401cd46a0feb0a61fd
Author: Lukasz Anforowicz <lukasza@chromium.org>
Date: Mon Feb 12 22:38:27 2018

XSDB: Log SiteIsolation.XSD.Browser.Blocked.ContentLength.* UMA metrics.

Bug:  804954 
Change-Id: I7991532d963f92aed50d92cc6492077a251a8b44
Reviewed-on: https://chromium-review.googlesource.com/907732
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Nick Carter <nick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536219}
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/content/browser/loader/cross_site_document_blocking_browsertest.cc
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/content/browser/loader/cross_site_document_resource_handler.cc
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/content/browser/loader/cross_site_document_resource_handler.h
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/content/browser/loader/cross_site_document_resource_handler_unittest.cc
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/content/test/data/site_isolation/nosniff.json-prefixed.js.mock-http-headers
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/content/test/data/site_isolation/nosniff.json.js.mock-http-headers
[modify] https://crrev.com/80693a3b5a96e9e390bafb401cd46a0feb0a61fd/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ef97565083ddfd89acd1c63ea1c4557d752fdd66

commit ef97565083ddfd89acd1c63ea1c4557d752fdd66
Author: Lukasz Anforowicz <lukasza@chromium.org>
Date: Thu Feb 22 16:54:50 2018

XSDB: Add ContentLengthWasZero to UKM data

Bug:  804954 
Change-Id: I4a6234c9ca9776e4b4885073bbfb3f0802e64e93
Reviewed-on: https://chromium-review.googlesource.com/920421
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538456}
[modify] https://crrev.com/ef97565083ddfd89acd1c63ea1c4557d752fdd66/content/browser/loader/cross_site_document_resource_handler.cc
[modify] https://crrev.com/ef97565083ddfd89acd1c63ea1c4557d752fdd66/tools/metrics/ukm/ukm.xml

I think gathering the data on Content-Length should be sufficient to mark this bug as fixed.  Also note that even responses with an empty body need to be "blocked" by stripping down their non-safelisted-headers.