Fixed by this commit:

commit bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb
Author: Tsuyoshi Horo <horo@chromium.org>
Date: Thu May 10 02:26:49 2018

Show the certificate request of signed exchange in DevTools.

We are trying to show SignedExchange related information in DevTools.
https://bit.ly/2HXVbCU

After this CL, the certificate request of signed exchange is displayed.

This CL depends on https://crrev.com/c/1029750

Bug:  830505 
Change-Id: Ia431b5bb889fa5f89754f11eb2e54dbf91d0e4fe
Reviewed-on: https://chromium-review.googlesource.com/1029755
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557431}
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/content/browser/devtools/render_frame_devtools_agent_host.cc
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/content/browser/devtools/render_frame_devtools_agent_host.h
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/content/browser/web_package/signed_exchange_cert_fetcher.cc
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/content/browser/web_package/signed_exchange_cert_fetcher.h
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/content/browser/web_package/signed_exchange_devtools_proxy.cc
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/content/browser/web_package/signed_exchange_devtools_proxy.h
[add] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/devtools/htxg/htxg-cert-not-found-expected.txt
[add] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/devtools/htxg/htxg-cert-not-found.js
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/devtools/htxg/htxg-navigation-expected.txt
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/devtools/htxg/htxg-navigation-expired-expected.txt
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/devtools/htxg/htxg-prefetch-expected.txt
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/devtools/htxg/htxg-prefetch-expired-expected.txt
[modify] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/loading/htxg/resources/README.md
[add] https://crrev.com/bb82ff6a3bb004b1c8e94410a4bcc1bfc1c67ecb/third_party/WebKit/LayoutTests/http/tests/loading/htxg/resources/htxg-cert-not-found.htxg