Make ProxyChain::proxy_server crash on multi-proxy chains

This will help to identify anywhere that assumes we are not using
multi-proxy chains, when in fact we are using them.

Bug: 1491092
Change-Id: I0240154a7a5219c0fb85fe474a740df0c6c63dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5034302
Commit-Queue: Dustin Mitchell <djmitche@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1230076}
NOKEYCHECK=True
GitOrigin-RevId: 95616e4071525092adfed1195c2f4a56fc8bd885
3 files changed