shill: cellular: Add mm1::ModemProxy::Drivers.

Upstream ModemManager has replaced the 'Driver' property of the
org.freedesktop.ModemManager1.Modem interface with a 'Drivers' proeprty.
This CL adds mm1::ModemProxy::Drivers after mm1::ModemProxy::Driver is
removed in the previous CL.

BUG=chromium-os:33901
TEST=Build and run unit tests.

Change-Id: I05e4db6198cd41ce6230ff5119fdeb09228bfd64
Reviewed-on: https://gerrit.chromium.org/gerrit/31423
Reviewed-by: Gary Morain <gmorain@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/mm1_modem_proxy.cc b/mm1_modem_proxy.cc
index 7dc3514..7649487 100644
--- a/mm1_modem_proxy.cc
+++ b/mm1_modem_proxy.cc
@@ -252,6 +252,15 @@
     return std::string();  // Make the compiler happy.
   }
 }
+const std::vector<std::string> ModemProxy::Drivers() {
+  SLOG(DBus, 2) << __func__;
+  try {
+    return proxy_.Drivers();
+  } catch (const DBus::Error &e) {
+    LOG(FATAL) << "DBus exception: " << e.name() << ": " << e.what();
+    return std::vector<std::string>();  // Make the compiler happy.
+  }
+}
 const std::string ModemProxy::Plugin() {
   SLOG(DBus, 2) << __func__;
   try {