The following code is documented to run on the UI Thread:

embedded_worker_instance.cc:SetupOnUIThread

https://cs.chromium.org/chromium/src/content/browser/service_worker/embedded_worker_instance.cc?l=93

This is clearly an error since the first statement is:
  DCHECK_CURRENTLY_ON(BrowserThread::UI);

The slow-reports (crash reports) reported executions on the IO Thread.

Examples:
f3916fb1906fb4d2
3095021d72ac6cbe
146cb32f29a28b31
2a290dfef60a7203
3210122c648db409
2b323e2747b96ff9
fdc7d8e0b725ede5
2b323e2747b96ff9
5bf47cc22f116da6
f398a80bb403849f
c8aa5a203fc860d2
8d39c5cde877f5cf
c93e1ede06e8ed0c
d04fac54ceac4a71
5a2014f88f4b4062