Findit identified the culprit r578971 with confidence 70.0% in the config "chromium.webkit / WebKit Win10"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVykgILEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCLbAWNocm9taXVtLndlYmtpdC9XZWJLaXQgV2luMTAvMzg1NjUvd2Via2l0X2xheW91dF90ZXN0cyBvbiBXaW5kb3dzLTEwLTE1MDYzL2RtbHlkSFZoYkM5dmRYUnZabUpzYVc1ckxXTnZjbk10Ym5NdlpYaDBaWEp1WVd3dmQzQjBMM05sY25acFkyVXRkMjl5YTJWeWN5OXpaWEoyYVdObExYZHZjbXRsY2k5dmNHRnhkV1V0Y21WemNHOXVjMlV0Y0hKbGJHOWhaR1ZrTG1oMGRIQnpMbWgwYld3PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20virtual/outofblink-cors-ns/external/wpt/service-workers/service-worker/opaque-response-preloaded.https.html&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVykgILEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCLbAWNocm9taXVtLndlYmtpdC9XZWJLaXQgV2luMTAvMzg1NjUvd2Via2l0X2xheW91dF90ZXN0cyBvbiBXaW5kb3dzLTEwLTE1MDYzL2RtbHlkSFZoYkM5dmRYUnZabUpzYVc1ckxXTnZjbk10Ym5NdlpYaDBaWEp1WVd3dmQzQjBMM05sY25acFkyVXRkMjl5YTJWeWN5OXpaWEoyYVdObExYZHZjbXRsY2k5dmNHRnhkV1V0Y21WemNHOXVjMlV0Y0hKbGJHOWhaR1ZrTG1oMGRIQnpMbWgwYld3PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b0f5b21b078c6382136d80d11f477822e6395e28

commit b0f5b21b078c6382136d80d11f477822e6395e28
Author: Peter Boström <pbos@chromium.org>
Date: Tue Jul 31 00:42:46 2018

Revert "Add MimeSniffingThrottle for navigation and requests from renderer"

This reverts commit 35be1bb5de7323013f3bc67e59f15be0d501dff0.

Reason for revert: Causing test flakes.

Bug:  chromium:869225 

Original change's description:
> Add MimeSniffingThrottle for navigation and requests from renderer
> 
> This CL adds MimeSniffingThrottle which can intercept the response in
> ThrottlingURLLoader. Typically mime sniffing happens in network::URLLoader and
> MimeSniffingThrottle is skipped in this case. If a request goes to a service
> worker or other interceptors, the MimeSniffngThrottle intercepts the response
> when a set of the mime type and the url is eligible to sniff the mime
> type. Sniffable mime types are defined in net::ShouldSniffMimeType(). When a
> mime type for performance sensitive components like "text/html",
> "text/javascript",or "text/css" is provided, it doesn't sniff.
> 
> Bug: 858975
> Cq-Include-Trybots: luci.chromium.try:linux_mojo
> Change-Id: Ib6e5b29867494fc0c0876952b4fec2a141288b36
> Reviewed-on: https://chromium-review.googlesource.com/1141744
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#578971}

TBR=kinuko@chromium.org,shimazu@chromium.org

Change-Id: I88721b279e5f38685b96d2d6f2813e423ff49540
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 858975
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1155501
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579259}
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/devtools/devtools_url_loader_interceptor.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/file_url_loader_factory.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/fileapi/file_system_url_loader_factory.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/fileapi/file_system_url_loader_factory_browsertest.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/loader/mime_sniffing_resource_handler.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/loader/mime_sniffing_resource_handler_unittest.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/loader/navigation_url_loader_impl.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/browser/loader/resource_dispatcher_host_unittest.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/common/BUILD.gn
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/common/DEPS
[delete] https://crrev.com/d35bcee05bb7136bd7a69d8e89d6fa2181185283/content/common/mime_sniffing_throttle.cc
[delete] https://crrev.com/d35bcee05bb7136bd7a69d8e89d6fa2181185283/content/common/mime_sniffing_throttle.h
[delete] https://crrev.com/d35bcee05bb7136bd7a69d8e89d6fa2181185283/content/common/mime_sniffing_throttle_unittest.cc
[delete] https://crrev.com/d35bcee05bb7136bd7a69d8e89d6fa2181185283/content/common/mime_sniffing_url_loader.cc
[delete] https://crrev.com/d35bcee05bb7136bd7a69d8e89d6fa2181185283/content/common/mime_sniffing_url_loader.h
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/common/service_worker/service_worker_loader_helpers.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/common/throttling_url_loader.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/common/throttling_url_loader.h
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/public/common/url_loader_throttle.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/public/common/url_loader_throttle.h
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/renderer/loader/resource_dispatcher.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/content/test/BUILD.gn
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/services/network/public/cpp/network_ipc_param_traits.h
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/services/network/public/cpp/resource_response.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/services/network/public/cpp/resource_response_info.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/services/network/public/cpp/resource_response_info.h
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/services/network/url_loader.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/services/network/url_loader_unittest.cc
[modify] https://crrev.com/b0f5b21b078c6382136d80d11f477822e6395e28/third_party/WebKit/LayoutTests/TestExpectations

 Issue 868804  has been merged into this issue.

Thanks for handling this. 
Let me see the flakiness dashboard for a while to check if the test calms down by the revert (c#3).

Dropping the sheriff label as this is being investigated.

There is no informative stack trace in the failing layout test's results.
I cannot reproduce this issue, and now I'm suspecting that 869378 is the same.
I'm still not sure when null deref on calling OnComplete() happen. Needs more investigation.

Okay, after relanding the patch with fix, this no longer crashes.
This was the same with 869378. 

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=external%2Fwpt%2Fservice-workers%2Fservice-worker%2Fopaque-response-preloaded.https.html%20