Allow 'chronos' user to call org.chromium.CrosDisks.EnumerateMountEntries.

BUG=chromium:356583
TEST=Tested the following:
1. Build and run unit tests.
2. Manually invoke org.chromium.CrosDisks.EnumerateMountEntries using
   dbus-send under the chronos user.

Change-Id: Ifc459a28e8b4148b2c892908d3a572aafe19ad41
Reviewed-on: https://chromium-review.googlesource.com/199688
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>
diff --git a/org.chromium.CrosDisks.conf b/org.chromium.CrosDisks.conf
index b1baa50..2a4f51f 100644
--- a/org.chromium.CrosDisks.conf
+++ b/org.chromium.CrosDisks.conf
@@ -26,6 +26,9 @@
            send_member="EnumerateDevices" />
     <allow send_destination="org.chromium.CrosDisks"
            send_interface="org.chromium.CrosDisks"
+           send_member="EnumerateMountEntries" />
+    <allow send_destination="org.chromium.CrosDisks"
+           send_interface="org.chromium.CrosDisks"
            send_member="Format" />
     <allow send_destination="org.chromium.CrosDisks"
            send_interface="org.chromium.CrosDisks"