Implement DeviceOrientationEvent constructor.

third_party/WebKit/Source/modules/device_orientation/DeviceOrientationEvent.idl

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-// https://w3c.github.io/deviceorientation/spec-source-orientation.html#deviceorientation
+// https://www.w3.org/TR/2016/CR-orientation-event-20160818/#deviceorientation
 
-// TODO(foolip): DeviceOrientationEvent should have a constructor.
+[Constructor(DOMString type, optional DeviceOrientationEventInit eventInitDict)]
 interface DeviceOrientationEvent : Event {
     readonly attribute double? alpha;
     readonly attribute double? beta;
     readonly attribute double? gamma;
     readonly attribute boolean absolute;
 
     // TODO(foolip): The init*Event() methods are not in the spec:
     // https://github.com/w3c/deviceorientation/issues/18
     [Measure] void initDeviceOrientationEvent([Default=Undefined] optional DOMString type,
                                               [Default=Undefined] optional boolean bubbles,
                                               [Default=Undefined] optional boolean cancelable,
                                               [Default=Undefined] optional double? alpha,
                                               [Default=Undefined] optional double? beta,
                                               [Default=Undefined] optional double? gamma,
                                               [Default=Undefined] optional boolean absolute);
 };