When using the [timeout attribute](https://xhr.spec.whatwg.org/#the-timeout-attribute) and an XHR
request times out, browsers trigger the `timeout` event (and execute the XHR's `ontimeout`
callback). Additionally, Safari 9 handles timed-out requests in the same way, even if no `timeout`
has been explicitly set on the XHR.
In the above cases, `HttpClient` would fail to capture the XHR's completing (with an error), so
the corresponding `Observable` would never complete.
PR Close#26453
PR Close#39807
61a0b6de6d