Get mdn_ from OwnNumbers property.

Accomodate gMock's need for a value to return from modem_proxy_->OwnNumbers().

BUG=None
TEST=Manual; use list-properties and confirm that Cellular.MDN matches
the "own" number from mmcli

Change-Id: I1db04e96e2f2ddbaba958263a41a1a66be643a4d
Reviewed-on: https://gerrit.chromium.org/gerrit/20256
Tested-by: Nathan J. Williams <njw@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: Nathan J. Williams <njw@chromium.org>
diff --git a/cellular_capability_universal.cc b/cellular_capability_universal.cc
index 8d8f44d..61e7acc 100644
--- a/cellular_capability_universal.cc
+++ b/cellular_capability_universal.cc
@@ -465,7 +465,10 @@
     }
   }
   if (mdn_.empty()) {
-    // TODO(jglasgow): use OwnNumbers()
+    // TODO(njw): Switch to asynchronous calls (crosbug.com/17583).
+    vector<string> numbers = modem_proxy_->OwnNumbers();
+    if (numbers.size() > 0)
+      mdn_ = numbers[0];
   }
   GetRegistrationState();
 }