Adds Server Previews protocol UMA plus UMA for not accepting previews

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 fb56385a4e4d3f615c1a29cf8e392dfd989c72b4..42d0b1e771ecb909316cea9fc7dc94acc7c955e6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10967,6 +10967,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="DataReductionProxy.Protocol.AcceptTransform"
+    enum="DataReductionProxyProtocolAcceptTransformEvent">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Records the sending of accepted transform headers to the data reduction
+    proxy and also receiving transforms and policies from the data reduction
+    proxy. Recorded on a per page request basis and possibly on a per image
+    subresource request basis for empty-image transforms.
+  </summary>
+</histogram>
+
+<histogram name="DataReductionProxy.Protocol.NotAcceptingTransform"
+    enum="DataReductionProxyProtocolNotAcceptingTransformReason">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Records the reason that a page request is not accepting proxy server
+    transforms. Recorded on a per page request basis.
+  </summary>
+</histogram>
+
 <histogram name="DataReductionProxy.ProxyPrefMigrationResult"
     enum="DataReductionProxyProxyPrefMigrationResult">
   <owner>sclittle@chromium.org</owner>