Remove custom per-request user-agent to HTTP proxies for CONNECTs.

When establishing an HTTPS connection over an HTTP/HTTPS/H2/QUIC proxy,
used the global User-Agent headers with the CONNECT request, instead of
using the custom User-Agent string associated with the original request.

Because of late binding, preconnects, and socket reuse, we could never
guarantee we'd actually use a tunnel creating using a particular
request's User-Agent header, and it's also a bit weird to have a side
channel between websites and the proxy.

The intent to deprecate and remove thread has some more details:
https://groups.google.com/a/chromium.org/forum/?utm_medium=email&utm_source=footer#!msg/blink-dev/Bm40gPAv4RE/Yt6r27pOBQAJ

[email protected]

Bug: 934325
Change-Id: I54b41a965b28df3a70bbe0c4c6ede3e709f7d548
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1487890
Reviewed-by: Matt Menke <[email protected]>
Reviewed-by: Tarun Bansal <[email protected]>
Reviewed-by: David Roger <[email protected]>
Reviewed-by: Misha Efimov <[email protected]>
Reviewed-by: Richard Coles <[email protected]>
Reviewed-by: Eric Roman <[email protected]>
Commit-Queue: Matt Menke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#638981}
diff --git a/net/socket/client_socket_pool_base_unittest.cc b/net/socket/client_socket_pool_base_unittest.cc
index b753e4c..dba6485 100644
--- a/net/socket/client_socket_pool_base_unittest.cc
+++ b/net/socket/client_socket_pool_base_unittest.cc
@@ -529,6 +529,7 @@
             nullptr /* client_socket_factory */,
             nullptr /* host_resolver */,
             nullptr /* proxy_delegate */,
+            nullptr /* http_user_agent_settings */,
             SSLClientSocketContext(),
             SSLClientSocketContext(),
             nullptr /* socket_performance_watcher_factory */,