Documenting the MobileTopToolbarVoiceButton action.

This was added in
https://chromium-review.googlesource.com/c/chromium/src/+/2496031 but
I'd missed updating actions.xml.

Bug: 1140581
Change-Id: Ie5139104f4a56aef0741f6ce4919bea5fefac896
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532767Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826580}

tools/metrics/actions/actions.xml

@@ -15554,6 +15554,14 @@ should be able to be added at any place in this file.
   </description>
 </action>
 
+<action name="MobileTopToolbarVoiceButton">
+  <owner>jds@chromium.org</owner>
+  <owner>chrome-language@google.com</owner>
+  <description>
+    The user tapped on the top toolbar's voice search button.
+  </description>
+</action>
+
 <action name="MobileUsingMenuByHwButtonDragging">
   <obsolete>
     Deprecated as of 06/2014. We removed this feature http://crbug.com/366154 .