Handle alternative endpoints in HTTP/2 and HTTP/3 cross-name pooling check

Treat service endpoints as eligible for cross-name pooling if they would
have been eligible for the ALPN protocol we're accessing. Additionally,
if we have no prior knowledge of a host supporting HTTP/3 (i.e. no
Alt-Svc), cross-name pooling requires the HTTP/3 in HTTPS-RR, to align
with the behavior on a new connection.

(Linked this to crbug.com/1264933 because that's where the TODO is
associated, but this is mostly post-HTTPS-RR cleanup and not directly
ECH-related.)

This allows us to remove the GetNonProtocolEndpoints helper function.

Bug: 1264933, 126134
Change-Id: If121b0e759b5e530def76057935f18861e3d064a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4516075


Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146719}