Add a DCHECK to ensure that probe responses are not checked for explicit bypasses

Add an explicit DCHECK in the data reduction proxy (DRP) to
ensure that we do not check for explicit bypasses
in the responses.

Bug: 760294
Change-Id: Ifd683efc79d484388f2446757e9025068c1d6fb8
Reviewed-on: https://chromium-review.googlesource.com/963100Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543452}

components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc

@@ -373,6 +373,12 @@ DataReductionProxyBypassType GetDataReductionProxyBypassType(
     DataReductionProxyInfo* data_reduction_proxy_info) {
   DCHECK(data_reduction_proxy_info);
 
+  // Responses from the warmup URL probe should not be checked for bypass types.
+  // Doing so may unnecessarily cause all data saver proxies to be marked as
+  // bad (e.g., when via header is missing on the response to the probe from the
+  // HTTP proxy).
+  DCHECK(url_chain.empty() || (params::GetWarmupURL() != url_chain.back()));
+
   bool has_via_header = HasDataReductionProxyViaHeader(headers, nullptr);
 
   if (has_via_header && HasURLRedirectCycle(url_chain)) {