[AF] Add metric enum values for the dynamic forms in AutofillFormEvents.

Change-Id: Ie5ed5f28bfdc5a7914dac544db17deac45e1f3b0
Reviewed-on: https://chromium-review.googlesource.com/1129044Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577171}

tools/metrics/histograms/enums.xml

@@ -2661,6 +2661,10 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="25"
       label="Submitted with fail-luhn-check card without selecting
              suggestions (once)"/>
+  <int value="26" label="The form changed dynamically"/>
+  <int value="27" label="The form changed dynamically and was fillable"/>
+  <int value="28" label="The form changed dynamically and was re-filled"/>
+  <int value="29" label="The form changed dynamically again after the refill"/>
 </enum>
 
 <enum name="AutofillFormSubmittedState">