ChromeFeaturesService: Allow root to check enabled features over D-Bus.

This simplifies tests and upstart scripts that check if features are
enabled.

BUG=chromium:915754

Change-Id: I35b5d1fb91a55dbde3e540a3d1dd8321dd12335a
Reviewed-on: https://chromium-review.googlesource.com/c/1440125Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Allen Webb <allenwebb@google.com>
Cr-Commit-Position: refs/heads/master@{#626796}

chrome/browser/chromeos/dbus/org.chromium.ChromeFeaturesService.conf

@@ -10,4 +10,10 @@
   <policy user="chronos">
     <allow own="org.chromium.ChromeFeaturesService"/>
   </policy>
+
+  <!-- upstart and tast run as root -->
+  <policy user="root">
+    <allow send_destination="org.chromium.ChromeFeaturesService"
+           send_interface="org.chromium.ChromeFeaturesServiceInterface"/>
+  </policy>
 </busconfig>