While navigating to a signed exchange, all signed exchange related DevTools messages are sent from the browser process.
 1) requestWillBeSent from NavigationRequest::OnStartChecksComplete() in browser process
 2) signedExchangeReceived from SignedExchangeHandler::OnCertVerifyComplete() in browser process
 3) requestWillBeSent from NavigationRequest::OnRedirectChecksComplete() in browser process
 4) responseReceived from NavigationRequest::OnResponseStarted() in browser process

But while prefetching a signed exchange files, only "2) signedExchangeReceived" message is sent from the browser process.
 1) requestWillBeSent from FrameFetchContext::DispatchWillSendRequest() in renderer process
 2) signedExchangeReceived from SignedExchangeHandler::OnCertVerifyComplete() in browser process
 3) requestWillBeSent from FrameFetchContext::DispatchWillSendRequest() in renderer process
 4) responseReceived from FrameFetchContext::DispatchDidReceiveResponse() in renderer process
So this message ordering is not guaranteed.

If SDK.NetworkDispatcher in NetworkManager.js receives the "2) signedExchangeReceived" message after "3) requestWillBeSent", DevTools can't correctly show the signed exchange information.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b48cdcc3aff8303d494bbe253a59a3e4696320bf

commit b48cdcc3aff8303d494bbe253a59a3e4696320bf
Author: Tsuyoshi Horo <horo@chromium.org>
Date: Mon Jun 11 08:06:41 2018

Mark prefetching Signed Exchange DevTools tests as flaky

Bug:  851363 
TBR: kouhei@chromium.org
Change-Id: Iffe66136447f801375716c7541940b7febb22304
Reviewed-on: https://chromium-review.googlesource.com/1094847
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565946}
[modify] https://crrev.com/b48cdcc3aff8303d494bbe253a59a3e4696320bf/third_party/WebKit/LayoutTests/TestExpectations

Test failures are processed as a part of a dedicated triage, bulk-closing the bugs.