Add UMA to histograms.xml for PLT.*_HTTPS_DataReductionProxy

BUG=374264

Review URL: https://codereview.chromium.org/884713004

Cr-Commit-Position: refs/heads/master@{#313637}

tools/metrics/histograms/histograms.xml

@@ -59536,6 +59536,9 @@ To add a new entry, add it with any value and run test to compute valid value.
 <histogram_suffixes name="DataReductionProxy">
   <suffix name="DataReductionProxy"
       label="Only page loads through the data reduction proxy are considered."/>
+  <suffix name="HTTPS_DataReductionProxy"
+      label="Only page loads through the data reduction proxy through an HTTP
+             tunnel for HTTPS origins are considered."/>
   <affected-histogram name="PLT.NT_Connect"/>
   <affected-histogram name="PLT.NT_DelayBeforeConnect"/>
   <affected-histogram name="PLT.NT_DelayBeforeDomainLookup"/>