[VK] Add VirtualKeyboard.Open to the history table.

Allows the metric to appear on the dashboard.
We're indexing by form factor because virtual keyboard usage depends
heavily on the form factor.

Bug: 752657
Change-Id: I7d32135c45fee05d86c3ce96b43831cc1ba648e3
Reviewed-on: https://chromium-review.googlesource.com/c/1353043Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612388}

tools/metrics/ukm/ukm.xml

@@ -5614,6 +5614,14 @@ be describing additional metrics about the same event.
       Type of a input field which the user is trying to use the Virtual Keyboard
       on.
     </summary>
+    <aggregation>
+      <history>
+        <index fields="profile.form_factor"/>
+        <statistics>
+          <enumeration/>
+        </statistics>
+      </history>
+    </aggregation>
   </metric>
 </event>