Copy Link to Text: Add label to histogram metric

Bug: 1105862
Change-Id: Ia79e07ca9ce44a8f95b20d70ddf4afe33901ee66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444171Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813338}

tools/metrics/histograms/enums.xml

@@ -12202,6 +12202,7 @@ Called by update_net_error_codes.py.-->
   <int value="17" label="Cut text"/>
   <int value="18" label="Paste text"/>
   <int value="19" label="Go to URL"/>
+  <int value="20" label="Copy link to text"/>
 </enum>
 
 <enum name="ContextMenuOptionIOS">