histograms.xml: add Platform.WiFiDeviceCount

BUG=chromium:758016
TEST=none

Change-Id: I6da6a2ebadabf39109534706fb8673b13ae95fff
Reviewed-on: https://chromium-review.googlesource.com/679274
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504767}

tools/metrics/histograms/histograms.xml

@@ -58766,6 +58766,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Platform.WiFiDeviceCount" units="wifi devices">
+  <owner>cernekee@chromium.org</owner>
+  <owner>semenzato@chromium.org</owner>
+  <summary>
+    Sample collected 10s after boot, showing how many WiFi interfaces are
+    present.
+  </summary>
+</histogram>
+
 <histogram name="Platform.WriteSectorsLong">
   <owner>gwendal@google.com</owner>
   <summary>