Creating this as I closed  issue 438941 .

According to the intent to ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/8vqcABTnDF4/xWq1rGGCBwAJ

requests with keepalive set works as follows in workers:
 1. The request will be rejected when there are too many inflight bytes (spec conformant).
 2. The request will be canceled when the worker is terminated (not spec conformant).

Marking Untriaged in case there's some existing bug this can be duped to.