Record UMA about the response's proxy_server in the DRP bypass logic.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index a679a18cd7afaa6c97a8a95283c6a176e5a7053a..a69c2ff46ee71b9067ee4ea271968bb90689d679 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -4470,6 +4470,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="DataReductionProxy.ResponseProxyServerStatus"
+    enum="DataReductionProxyResponseProxyServerStatus">
+  <owner>sclittle@chromium.org</owner>
+  <owner>bengr@chromium.org</owner>
+  <summary>
+    Reports the state of the proxy server field in the response seen by the Data
+    Reduction Proxy bypass logic.
+  </summary>
+</histogram>
+
 <histogram name="DataReductionProxy.SettingsConversion"
     enum="DataReductionProxySettingsConversion">
   <obsolete>
@@ -49381,6 +49391,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="3" label="Enabled from second screen"/>
 </enum>
 
+<enum name="DataReductionProxyResponseProxyServerStatus" type="int">
+  <int value="0" label="Empty proxy server"/>
+  <int value="1" label="DRP proxy server"/>
+  <int value="2" label="Non-DRP proxy server without DRP via header"/>
+  <int value="3" label="Non-DRP proxy server with DRP via header"/>
+</enum>
+
 <enum name="DataReductionProxySettingsConversion" type="int">
   <int value="0" label="OFF to OFF"/>
   <int value="1" label="OFF to ON"/>