Add activated attribute.

index.bs

@@ -593,6 +593,7 @@ A [=sensor=] has an associated <dfn>current polling frequency</dfn> which is ini
 [SecureContext]
 interface Sensor : EventTarget {
   readonly attribute DOMHighResTimeStamp? timestamp;
+  readonly attribute Boolean activated;
   void start();
   void stop();
   attribute EventHandler onchange;
@@ -681,6 +682,17 @@ with the internal slots described in the following table:
 The getter of the {{Sensor/timestamp!!attribute}} attribute returns
 [=latest reading=]["timestamp"].
 
+### Sensor.activated ### {#sensor-activated}
+
+<div algorithm="is sensor activated">
+    The getter of the {{Sensor/activated!!attribute}} attribute must run these steps
+    or their [=equivalent=]:
+
+    1.  If <emu-val>this</emu-val>.{{[[state]]}} is "activating",
+        return `true`.
+    1.  Otherwise, return `false`.
+</div>
+
 ### Sensor.start() ### {#sensor-start}
 
 <div algorithm="to start a sensor">