shill: expose Manager.ClaimedDevices property

Expose a ClaimedDevices property on Manager which contains a list of the
names of every device claimed by the current DeviceClaimer.  If no
DeviceClaimer exists, then returned list will be empty.

BUG=chrome-os-partner:39112
TEST=FEATURES=test emerge-amd64-generic shill
CQ-DEPEND=CL:265737

Change-Id: I0f534dade7006fcb92492a425511703377fc1094
Reviewed-on: https://chromium-review.googlesource.com/265739
Reviewed-by: Zeping Qiu <zqiu@chromium.org>
Commit-Queue: Garret Kelly <gdk@chromium.org>
Tested-by: Garret Kelly <gdk@chromium.org>
diff --git a/device_claimer.h b/device_claimer.h
index 543105f..e646842 100644
--- a/device_claimer.h
+++ b/device_claimer.h
@@ -52,6 +52,10 @@
 
   virtual bool default_claimer() const { return default_claimer_; }
 
+  const std::set<std::string> &claimed_device_names() const {
+    return claimed_device_names_;
+  }
+
  private:
   // DBus name watcher for monitoring the DBus service of the claimer.
   std::unique_ptr<DBusNameWatcher> dbus_name_watcher_;