Track metrics for the Billing Interstitial block list

Bug: 867518
Change-Id: Ic6f4f6db89960c3ccdeeb3d8590ace98a8545ed7
Reviewed-on: https://chromium-review.googlesource.com/c/1253061Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595927}

tools/metrics/histograms/histograms.xml

@@ -130930,6 +130930,7 @@ uploading your change for review.
   <suffix name="ChromeFilenameClientIncident" label="(Obsolete, not recorded)"/>
   <suffix name="ChromeUrlClientIncident"/>
   <suffix name="IpMalware"/>
+  <suffix name="UrlBilling"/>
   <suffix name="UrlCsdDownloadWhitelist"/>
   <suffix name="UrlCsdWhitelist"/>
   <suffix name="UrlMalBin"/>