Bug 1722942 use beConservative for proxy failover to direct r=dragana,necko-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D121234

netwerk/protocol/http/nsHttpChannel.cpp

@@ -2652,9 +2652,8 @@ nsresult nsHttpChannel::ProxyFailover() {
       return rv;
     }
     // If this request used a failed proxy and there is no failover available,
-    // fallback to DIRECT connections for system principal requests.
-    if (mLoadInfo->GetLoadingPrincipal() &&
-        mLoadInfo->GetLoadingPrincipal()->IsSystemPrincipal()) {
+    // fallback to DIRECT connections for conservative requests.
+    if (LoadBeConservative()) {
       rv = pps->NewProxyInfo("direct"_ns, ""_ns, 0, ""_ns, ""_ns, 0, UINT32_MAX,
                              nullptr, getter_AddRefs(pi));
     }

toolkit/components/extensions/test/xpcshell/test_proxy_failover.js

@@ -42,8 +42,9 @@ function getBadProxyPort() {
 
 function xhr(url) {
   return new Promise((resolve, reject) => {
-    let req = new XMLHttpRequest({ mozSystem: true });
+    let req = new XMLHttpRequest();
     req.open("GET", `${url}?t=${Math.random()}`);
+    req.channel.QueryInterface(Ci.nsIHttpChannelInternal).beConservative = true;
     req.onload = () => {
       resolve(req.responseText);
     };