Added Login.UsersActiveWeekly UMA metric description to histograms.xml.

BUG=365352
TEST=none
NOCHECK=true

Review URL: https://codereview.chromium.org/303423003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274075 0039d316-1c4b-4281-b951-d872f2087c98

tools/metrics/histograms/histograms.xml

@@ -9266,6 +9266,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>Chrome OS login success reason.</summary>
 </histogram>
 
+<histogram name="Login.UsersActiveWeekly" units="users">
+  <owner>alemate@chromium.org</owner>
+  <owner>nkostylev@chromium.org</owner>
+  <summary>
+    Chrome OS histogram that keeps track of number of users who have logged in
+    in the last 7 days. Reported on every boot and once a day after that.
+  </summary>
+</histogram>
+
 <histogram name="Login.UserType" enum="LoginUserType">
   <owner>cmasone@chromium.org</owner>
   <summary>