Update histograms.xml for UseCounters.

This updates histograms.xml for the UseCounter added in
https://codereview.chromium.org/1017903003

BUG=none

Review URL: https://codereview.chromium.org/1010083005

Cr-Commit-Position: refs/heads/master@{#322023}

tools/metrics/histograms/histograms.xml

@@ -50430,6 +50430,7 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="736" label="CryptoAlgorithmEcdh"/>
   <int value="737" label="CryptoAlgorithmHkdf"/>
   <int value="738" label="CryptoAlgorithmPbkdf2"/>
+  <int value="739" label="DocumentSetDomain"/>
 </enum>
 
 <enum name="FFmpegCodecs" type="int">