Fix missing user action for MobileOmniboxLens

- This was missed in a previous CL [1]
- It prevents anyone else adding new metrics until it is resolved.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/2693505

(cherry picked from commit f62431e)

Bug: None
Test: None
Change-Id: I875b97755bcbaa14bcd3c8e261a23d7b35bd81f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2719656
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Auto-Submit: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#858202}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2740295
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/branch-heads/4430@{#170}
Cr-Branched-From: e5ce7dc-refs/heads/master@{#857950}

tools/metrics/actions/actions.xml

@@ -14606,6 +14606,12 @@ should be able to be added at any place in this file.
   </description>
 </action>
 
+<action name="MobileOmniboxLens">
+  <owner>yusuyoutube@google.com</owner>
+  <owner>benwgold@google.com</owner>
+  <description>Mobile Omnibox Lens button clicked.</description>
+</action>
+
 <action name="MobileOmniboxRefineSuggestion">
   <obsolete>
     Deprecated 07/2017. Replaced by MobileOmniboxRefineSuggestion.Query and