The cancelled request is followed by a fulfilled request for the same URL. I see CreateURLLoader gets called twice. In the first load, the subresource loader returned net::OK.

[1:1:1009/155905.277834:ERROR:service_worker_network_provider.cc(102)] virtual std::unique_ptr<blink::WebURLLoader> content::(anonymous namespace)::WebServiceWorkerNetworkProviderForFrame::CreateURLLoader(const blink::WebURLRequest &, std::unique_ptr<blink::scheduler::WebResourceLoadingTaskRunnerHandle>): https://d3l4lslmo9uf22.cloudfront.net/r/14048/video-hls-v500008.ts
[1:4:1009/155905.278154:ERROR:service_worker_subresource_loader.cc(171)] content::ServiceWorkerSubresourceLoader::ServiceWorkerSubresourceLoader(network::mojom::URLLoaderRequest, int32_t, int32_t, uint32_t, const network::ResourceRequest &, network::mojom::URLLoaderClientPtr, const net::MutableNetworkTrafficAnnotationTag &, scoped_refptr<content::ControllerServiceWorkerConnector>, scoped_refptr<network::SharedURLLoaderFactory>, scoped_refptr<base::SequencedTaskRunner>)https://d3l4lslmo9uf22.cloudfront.net/r/14048/video-hls-v500008.ts
[1:4:1009/155905.278598:ERROR:service_worker_subresource_loader.cc(496)] void content::ServiceWorkerSubresourceLoader::CommitCompleted(int)https://d3l4lslmo9uf22.cloudfront.net/r/14048/video-hls-v500008.ts: 0
[1:1:1009/155905.278904:ERROR:service_worker_network_provider.cc(102)] virtual std::unique_ptr<blink::WebURLLoader> content::(anonymous namespace)::WebServiceWorkerNetworkProviderForFrame::CreateURLLoader(const blink::WebURLRequest &, std::unique_ptr<blink::scheduler::WebResourceLoadingTaskRunnerHandle>): https://d3l4lslmo9uf22.cloudfront.net/r/14048/video-hls-v500008.ts

CreateURLLoader gets called twice also in non-S13N, but there's no cancelled output in DevTools:

[1:1:1009/164859.619140:ERROR:service_worker_network_provider.cc(102)] virtual std::unique_ptr<blink::WebURLLoader> content::(anonymous namespace)::WebServiceWorkerNetworkProviderForFrame::CreateURLLoader(const blink::WebURLRequest &, std::unique_ptr<blink::scheduler::WebResourceLoadingTaskRunnerHandle>): https://d3l4lslmo9uf22.cloudfront.net/r/14048/video-hls-v500022.ts
[1:1:1009/164859.621103:ERROR:service_worker_network_provider.cc(102)] virtual std::unique_ptr<blink::WebURLLoader> content::(anonymous namespace)::WebServiceWorkerNetworkProviderForFrame::CreateURLLoader(const blink::WebURLRequest &, std::unique_ptr<blink::scheduler::WebResourceLoadingTaskRunnerHandle>): https://d3l4lslmo9uf22.cloudfront.net/r/14048/video-hls-v500022.ts

Also if I set a breakpoint in the service worker's fetch event, it only gets hit once for the main resource request. Subsequent requests don't stop at the breakpoint even though the service worker is running.

The service worker returns a "fallback was required" response. I discovered in non-S13nSW these requests are suppressed by matching on the HTTP response status line "Service Worker Fallback Required" via  issue 477685 . In S13nSW we aren't adding this line.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b6bd3f0297d8ebb3e76908e83295cb17fdffcc92

commit b6bd3f0297d8ebb3e76908e83295cb17fdffcc92
Author: Matt Falkenhagen <falken@chromium.org>
Date: Thu Oct 11 09:43:19 2018

service worker: s13n: Don't show CORS fallback responses in DevTools.

DevTools recognizes the string "Service Worker Fallback Required"
(see  issue 477685 ) and doesn't show the response in that case.

Bug:  893459 
Change-Id: Ieadd2d1ce3e738b866ae78d20396c36fb8880c75
Reviewed-on: https://chromium-review.googlesource.com/c/1276065
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598709}
[modify] https://crrev.com/b6bd3f0297d8ebb3e76908e83295cb17fdffcc92/content/renderer/service_worker/service_worker_subresource_loader.cc
[modify] https://crrev.com/b6bd3f0297d8ebb3e76908e83295cb17fdffcc92/content/renderer/service_worker/service_worker_subresource_loader_unittest.cc

Fixed in M71.