I see two kinds of failures:

1) Frequent: inside SW code:
When an extension is uninstalled, we hit this DCHECK

void ServiceWorkerNewScriptLoader::OnComplete(
    const network::URLLoaderCompletionStatus& status) {
  DCHECK(network_loader_state_ == NetworkLoaderState::kWaitingForBody ||
         network_loader_state_ == NetworkLoaderState::kLoadingBody);
...

The code that serves the script in extension_protocols.cc is:

ExtensionURLLoaderFactory::CreateLoaderAndStart() ..

    base::FilePath directory_path;
    if (!GetDirectoryForExtensionURL(request.url, extension_id, extension.get(),
                                     registry->disabled_extensions(),
                                     &directory_path)) {
      client->OnComplete(network::URLLoaderCompletionStatus(net::ERR_FAILED));
      return;
    }

I *think* client->OnComplete is throwing off ServiceWorkerNewScriptLoader.
network_loader_state_ is NetworkLoaderState::kLoadingHeader at that point.

I'm speculating the DCHECK can come after once there is no error in OnComplete.
SW folks can tell better, I'll post a patch to discuss.

2) Infrequent:
StartTestFromBackgroundPage in service_worker_apitest.cc:
content::ExecuteSCriptAndExtractString doesn't return and
the test times out.

This sounds like a common pattern used in the test file and would likely
contribute to other test flakiness in the test suite, will follow up on
that...

Yes we also are tracking that DCHECK failure at issue 868183.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6a3826e7d08c95991184aac451ecbc63b6967412

commit 6a3826e7d08c95991184aac451ecbc63b6967412
Author: Istiaque Ahmed <lazyboy@chromium.org>
Date: Fri Aug 24 06:52:46 2018

Fix error handling in ServiceWorkerNewScriptLoader::OnComplete

In case of an error where service worker script loading fails (e.g.
when there is an attempt to load an extension service worker script
after the extension has been uninstalled), the current code
fails DCHECK if network stack calls OnComplete with error param
right from mojom::URLLoaderFactory::CreateLoaderAndStart.

This CL changes the DCHECK to apply only for non-error case.
This should fix the flakiness of the test:
ServiceWorkerTest.ServiceWorkerSuspensionOnExtensionUnload.


Bug:  877218 , 868183
Test: Repeated aforementioned test x 500 locally, no DCHECK observed.
Change-Id: I1691771e466365477051f5ca1136f01f921a1ec8
Reviewed-on: https://chromium-review.googlesource.com/1187762
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585730}
[modify] https://crrev.com/6a3826e7d08c95991184aac451ecbc63b6967412/content/browser/service_worker/service_worker_new_script_loader.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6475f54b562f9067a6879f122e53d8ed55e3f842

commit 6475f54b562f9067a6879f122e53d8ed55e3f842
Author: Istiaque Ahmed <lazyboy@chromium.org>
Date: Tue Aug 28 04:20:21 2018

[Extension SW] Fix flakiness of tests calling StartTestFromBackgroundPage

StartTestFromBackgroundPage opens an extension background page
and then calls ExectuteScript*. It is possible that the function
we call through ExectuteScript isn't defined yet. This can cause
tests to time out.

Let the background page send a "ready" message so that browser_tests
can wait for it before proceeding to call ExectuteScript*.

Bug:  877218 
Change-Id: I637a2b69229c78dd645b7b078197b15015bf1b60
Reviewed-on: https://chromium-review.googlesource.com/1192352
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586577}
[modify] https://crrev.com/6475f54b562f9067a6879f122e53d8ed55e3f842/chrome/browser/extensions/service_worker_apitest.cc
[modify] https://crrev.com/6475f54b562f9067a6879f122e53d8ed55e3f842/chrome/test/data/extensions/api_test/service_worker/background/background.js