[Autofill Assistant] Add missing entry to Histogram

Bug: b/159309621
Change-Id: I45016405c85a5bef1b674ea8477c34b5bbc30711
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306251Reviewed-by: Luca Hunkeler <hluca@google.com>
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#789862}

tools/metrics/histograms/enums.xml

@@ -4292,6 +4292,7 @@ Unknown properties are collapsed to zero. -->
   <int value="21" label="User navigation to a different domain in BROWSE mode"/>
   <int value="22" label="User clicked back button"/>
   <int value="23" label="User clicked back button during onboarding"/>
+  <int value="24" label="User navigatioin in RUNNING state"/>
 </enum>
 
 <enum name="AutofillAssistantFeatureModuleInstallation">