Make ProxyDelegate work in TestURLRequestContext.

Bug: 915659
Change-Id: Ib277ee462b14fbeba96b065149ea413461a51cda
Reviewed-on: https://chromium-review.googlesource.com/c/1448154Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627965}

net/url_request/url_request_test_util.cc

@@ -131,6 +131,7 @@ void TestURLRequestContext::Init() {
     session_context.ct_policy_enforcer = ct_policy_enforcer();
     session_context.transport_security_state = transport_security_state();
     session_context.proxy_resolution_service = proxy_resolution_service();
+    session_context.proxy_delegate = proxy_delegate();
     session_context.ssl_config_service = ssl_config_service();
     session_context.http_auth_handler_factory = http_auth_handler_factory();
     session_context.http_server_properties = http_server_properties();