Adds UMA to measure when the data reduction proxy via header is missing

components/data_reduction_proxy/browser/data_reduction_proxy_protocol.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/data_reduction_proxy/browser/data_reduction_proxy_protocol.h"
 
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 #include "components/data_reduction_proxy/browser/data_reduction_proxy_params.h"
 #include "components/data_reduction_proxy/browser/data_reduction_proxy_tamper_detection.h"
@@ skipping 47 matching lines @@
     return false;
 
   // Empty implies either that the request was served from cache or that
   // request was served directly from the origin.
   if (request->proxy_server().IsEmpty())
     return false;
 
   if (data_reduction_proxy_type_info.proxy_servers.first.is_empty())
     return false;
 
+  // At this point, the response is expected to have the data reduction proxy
+  // via header, so detect and report cases where the via header is missing.
+  DataReductionProxyUsageStats::DetectAndRecordMissingViaHeaderResponseCode(
+      !data_reduction_proxy_type_info.proxy_servers.second.is_empty(),
+      original_response_headers);
+
   DataReductionProxyTamperDetection::DetectAndReport(
       original_response_headers,
       data_reduction_proxy_type_info.proxy_servers.first.SchemeIsSecure());
 
   DataReductionProxyInfo data_reduction_proxy_info;
   DataReductionProxyBypassType bypass_type =
       GetDataReductionProxyBypassType(original_response_headers,
                                       &data_reduction_proxy_info);
   if (proxy_bypass_type)
     *proxy_bypass_type = bypass_type;
@@ skipping 119 matching lines @@
   net::ProxyService* proxy_service = request->context()->proxy_service();
   DCHECK(proxy_service);
 
   proxy_service->MarkProxiesAsBadUntil(proxy_info,
                                        bypass_duration,
                                        fallback,
                                        request->net_log());
 }
 
 }  // namespace data_reduction_proxy