Don't preresolve DNS if a fixed proxy configuration is in place.

net/proxy/proxy_service.cc

Index: net/proxy/proxy_service.cc
diff --git a/net/proxy/proxy_service.cc b/net/proxy/proxy_service.cc
index 4c59024746da2662e643bf8f4568d0c9a124d645..cbb0c6a627b6aee11c8891d35f5f952e785d026b 100644
--- a/net/proxy/proxy_service.cc
+++ b/net/proxy/proxy_service.cc
@@ -980,7 +980,6 @@ int ProxyService::ResolveProxy(const GURL& raw_url,
                                NetworkDelegate* network_delegate,
                                const BoundNetLog& net_log) {
   DCHECK(CalledOnValidThread());
-  DCHECK(!callback.is_null());
 
   net_log.BeginEvent(NetLog::TYPE_PROXY_SERVICE);
 
@@ -1005,6 +1004,9 @@ int ProxyService::ResolveProxy(const GURL& raw_url,
     return DidFinishResolvingProxy(url, load_flags, network_delegate,
                                    result, rv, net_log);
 
+  if (callback.is_null())
+    return ERR_IO_PENDING;

[eroman, 2014/09/23 20:50:42]

This calling contract is confusing, since the request is not really pending in
this case (it will never be started).

You can extract this as a helper function, and then have two entry points:
  (1) The existing: int ResolveProxy(...)
  (2) A new: bool TryResolveProxySynchronously(...)

In the case of ResolveProxy() passing a null callback should remain an error,
whereas TryResolveProxySynchronously won't even take a callback.

[bemasc, 2014/09/24 18:34:37]

Done.

+
   scoped_refptr<PacRequest> req(
       new PacRequest(this, url, load_flags, network_delegate,
                      result, callback, net_log));