Use URLRequestContextBuilderMojo to create the SystemURLRequestContext.

net/proxy/dhcp_proxy_script_fetcher.cc

Index: net/proxy/dhcp_proxy_script_fetcher.cc
diff --git a/net/proxy/dhcp_proxy_script_fetcher.cc b/net/proxy/dhcp_proxy_script_fetcher.cc
index 7c381cac1c2ed4fb1fec19e985394c178a7d684f..839c59bbd31f8f36965738e3bd019a2b71ee811c 100644
--- a/net/proxy/dhcp_proxy_script_fetcher.cc
+++ b/net/proxy/dhcp_proxy_script_fetcher.cc
@@ -33,4 +33,8 @@ const GURL& DoNothingDhcpProxyScriptFetcher::GetPacURL() const {
   return gurl_;
 }
 
+std::string DoNothingDhcpProxyScriptFetcher::GetFetcherName() const {
+  return "do nothing";
+}
+
 }  // namespace net