Bug 1523562 [wpt PR 15094] - Update interfaces/ambient-light.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/ambient-light.idl (#15094)

Source: https://github.com/tidoust/reffy-reports/blob/71768cc/whatwg/idl/ambient-light.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/484671880
--

wpt-commits: 03d9fb9a2434b715a30c0440125fb55ce5df8d1d
wpt-pr: 15094

testing/web-platform/tests/interfaces/ambient-light.idl

@@ -7,3 +7,7 @@
 interface AmbientLightSensor : Sensor {
   readonly attribute double? illuminance;
 };
+
+dictionary AmbientLightReadingValues {
+  required double? illuminance;
+};