Adds Server Previews protocol UMA plus UMA for not accepting previews

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index fda1ce096b0d23fe10823453f76d7f27f851b9e5..4b313e99c12b8c9c5e4ca7ae440c00d10796b7df 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -7053,6 +7053,25 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="3" label="Enabled from second screen"/>
 </enum>
 
+<enum name="DataReductionProxyProtocolAcceptTransformEvent">
+  <int value="0" label="AcceptTransform lite-page sent"/>
+  <int value="1" label="ContentTransform lite-page received"/>
+  <int value="2" label="Page policy for empty-image received"/>
+  <int value="3" label="AcceptTransform empty-image sent"/>
+  <int value="4" label="ContentTransform empty-image received"/>
+  <int value="5" label="AcceptTransform compressed-video sent"/>
+</enum>
+
+<enum name="DataReductionProxyProtocolNotAcceptingTransformReason">
+  <int value="0"
+      label="Not accepting proxy transforms, transforms disabled on client"/>
+  <int value="1"
+      label="Not accepting proxy transforms, page blacklisted on client"/>
+  <int value="2"
+      label="Not accepting proxy transforms, not on cellular in cellular-only
+             mode"/>
+</enum>
+
 <enum name="DataReductionProxyProxyPrefMigrationResult">
   <int value="0" label="Proxy pref not cleared"/>
   <int value="1" label="Empty proxy pref was cleared"/>