Add "Redirect Found" entry in NetworkPortalResult histogram

We plan to add a new portal result (Redirect Found) in CL:1532970.
Modify the NetworkPortalResult histogram to accommodate this.

Bug: 903645
Change-Id: I488508ed6f618d69baa0f59a8b7f28d52c8c8fac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542660Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Matthew Wang <matthewmwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646489}

tools/metrics/histograms/enums.xml

@@ -39779,6 +39779,7 @@ Called by update_net_trust_anchors.py.-->
   <int value="7" label="Content Failure"/>
   <int value="8" label="Content Timeout"/>
   <int value="9" label="Unknown"/>
+  <int value="10" label="Redirect Found"/>
 </enum>
 
 <enum name="NetworkProblemType">