Record errors that trigger a data reduction proxy bypass

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 cff311bc69781a2a8cb460c5c7b242a61804c7f0..1906cbe9b7fe94dd1410736aa48178f216a535ac 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2675,6 +2675,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="DataReductionProxy.BypassOnNetworkErrorFallback"
+    enum="NetErrorCodes">
+  <owner>bengr@chromium.org</owner>
+  <summary>
+    Positive net error code that caused a the fallback data reduction proxy to

[Alexei Svitkine (slow), 2014/05/26 17:25:08]

Nit: "a the" -> "the"

[bengr, 2014/05/27 15:59:54]

Done.

+    be bypassed an put on the proxy retry list.

[Alexei Svitkine (slow), 2014/05/26 17:25:08]

Nit: "an" -> "and"

[bengr, 2014/05/27 15:59:54]

Done.

+  </summary>

[Alexei Svitkine (slow), 2014/05/26 17:25:08]

Nit: mention when this is logged - is it when the error occurs or something
else? Same below.

[bengr, 2014/05/27 15:59:54]

Done.

+</histogram>
+
+<histogram name="DataReductionProxy.BypassOnNetworkErrorPrimary"
+    enum="NetErrorCodes">
+  <owner>bengr@chromium.org</owner>
+  <summary>
+    Positive net error code that caused a the primary data reduction proxy to be

[Alexei Svitkine (slow), 2014/05/26 17:25:08]

Nit: "a the" -> "the"

[bengr, 2014/05/27 15:59:54]

Done.

+    bypassed an put on the proxy retry list.

[Alexei Svitkine (slow), 2014/05/26 17:25:08]

Nit: "an" -> "and"

[bengr, 2014/05/27 15:59:54]

Done.

+  </summary>
+</histogram>
+
 <histogram name="DataReductionProxy.ProbeURL"
     enum="DataReductionProxyProbeURLFetchResult">
   <owner>bengr@chromium.org</owner>
@@ -32393,6 +32411,7 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="2" label="Bypass due to internal server error"/>
   <int value="3" label="Bypass due to other error"/>
   <int value="4" label="Bypass due to missing via header"/>
+  <int value="5" label="Bypass due to 4xx response"/>
 </enum>
 
 <enum name="DataReductionProxyProbeURLFetchResult" type="int">