Record errors that trigger a data reduction proxy bypass

net/http/http_response_headers.cc

Index: net/http/http_response_headers.cc
diff --git a/net/http/http_response_headers.cc b/net/http/http_response_headers.cc
index 7ecf37a2352704f02685e9012c64a04c296999bd..b2248ec3c48afd8c64fbf16b2a033a5e1f07d98a 100644
--- a/net/http/http_response_headers.cc
+++ b/net/http/http_response_headers.cc
@@ -1504,6 +1504,15 @@ HttpResponseHeaders::GetDataReductionProxyBypassEventType(
     // response is to minimize information transfer, a sender in general
     // should not generate representation metadata other than Cache-Control,
     // Content-Location, Date, ETag, Expires, and Vary.
+
+    // A Via header might also not be present in a 4xx response. If there is
+    // evidence that the 4xx response was generated by the proxy, bypass it.
+    if (response_code() >= HTTP_BAD_REQUEST &&
+        response_code() < HTTP_INTERNAL_SERVER_ERROR) {
+      if (HasHeaderValue("Server", "GFE/2.0"))
+        return ProxyService::PROXY_4XX_BYPASS;
+      // Otherwise, bypass on the missing Via header.
+    }
     return ProxyService::MISSING_VIA_HEADER;
   }
   // There is no bypass event.