Added new enum value for FirebaseConfiguredState

In http://crrev/c/1272598, a new enumerated value for
FirebaseConfiguredState was added. This adds to enums.xml for display
in frontend tools.

Bug: 892880
Change-Id: I1d3794ef4fdd3c0eaa824e34f43afd2bc0f9c41b
Reviewed-on: https://chromium-review.googlesource.com/c/1297450Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602218}

tools/metrics/histograms/enums.xml

@@ -21689,6 +21689,7 @@ Called by update_net_error_codes.py.-->
   <int value="1" label="Enabled, at First Run"/>
   <int value="2" label="Enabled, not at First Run"/>
   <int value="3" label="Disabled, Conversion Window expired"/>
+  <int value="4" label="Disabled, Legacy User upgraded"/>
 </enum>
 
 <enum name="FirstMeaningfulPaintOrdering">