cros_system_api: Add constants for valid parameters of the SetLEConnectionParameters API.

BUG=chromium:725367
TEST=N/A

Change-Id: Ic50b8c1cf4ab049ac6b058273dbc3a892d77771f
Reviewed-on: https://chromium-review.googlesource.com/516551
Commit-Ready: Tim Song <tengs@chromium.org>
Tested-by: Tim Song <tengs@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Miao-chen Chou <mcchou@chromium.org>
diff --git a/dbus/service_constants.h b/dbus/service_constants.h
index 0c8c524..73e5ef2 100644
--- a/dbus/service_constants.h
+++ b/dbus/service_constants.h
@@ -234,6 +234,12 @@
 // Bluetooth Device plugin methods.
 const char kGetConnInfo[] = "GetConnInfo";
 const char kSetLEConnectionParameters[] = "SetLEConnectionParameters";
+// Valid connection parameters that can be passed to the
+// SetLEConnectionParameters API as dictionary keys.
+const char kLEConnectionParameterMinimumConnectionInterval[] =
+    "MinimumConnectionInterval";
+const char kLEConnectionParameterMaximumConnectionInterval[] =
+    "MaximumConnectionInterval";
 }  // namespace bluetooth_plugin_device
 
 namespace bluetooth_adapter {