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

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 365d7b2085b72bec65c86a97344e3c4c834e3336..a6d3bc605f75296ba44394290e90b6e192b49f58 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3557,6 +3557,50 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="DataReductionProxy.MissingViaHeader4xxResponseBytes"
+    units="bytes">
+  <owner>bengr@chromium.org</owner>
+  <owner>sclittle@chromium.org</owner>
+  <summary>
+    Counts the response bytes of responses with 4xx response codes that Chrome
+    expected to come through a data reduction proxy, but the response did not
+    have the data reduction proxy via header. Note that this does not include
+    responses that were bypassed or responses that trigger bypasses.
+  </summary>
+</histogram>
+
+<histogram name="DataReductionProxy.MissingViaHeaderOtherResponseBytes"
+    units="bytes">
+  <owner>bengr@chromium.org</owner>
+  <owner>sclittle@chromium.org</owner>
+  <summary>
+    Counts the response bytes of responses with non-4xx response codes that
+    Chrome expected to come through a data reduction proxy, but the response did
+    not have the data reduction proxy via header. Note that this does not
+    include responses that were bypassed or responses that trigger bypasses.
+  </summary>
+</histogram>
+
+<histogram name="DataReductionProxy.MissingViaHeaderResponseCodeFallback">
+  <owner>bengr@chromium.org</owner>
+  <owner>sclittle@chromium.org</owner>
+  <summary>
+    Counts the different HTTP response codes of responses that Chrome expected
+    to come through the fallback data reduction proxy, but where the response
+    did not have the data reduction proxy via header.
+  </summary>
+</histogram>
+
+<histogram name="DataReductionProxy.MissingViaHeaderResponseCodePrimary">
+  <owner>bengr@chromium.org</owner>
+  <owner>sclittle@chromium.org</owner>
+  <summary>
+    Counts the different HTTP response codes of responses that Chrome expected
+    to come through the primary data reduction proxy, but where the response did
+    not have the data reduction proxy via header.
+  </summary>
+</histogram>
+
 <histogram name="DataReductionProxy.NetworkChangeEvents"
     enum="DataReductionProxyNetworkChangeEvent">
   <owner>bengr@chromium.org</owner>