When we add a timer, make sure that the network poller is initialized,
since we will use it if we have to wait for the timer to be ready.

Updates #27707

Change-Id: I0637fe646bade2cc5ce50b745712292aa9c445b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/171830
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>