[Sensors] Implement Sensor.onactivate

third_party/WebKit/Source/modules/sensor/Sensor.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Specification at:
 // https://w3c.github.io/sensors/#sensor0
 
 enum SensorState {
   "idle",
   "activating",
   "active",
   "errored"
 };
 
 [
     ActiveScriptWrappable,
     DependentLifetime,
     RuntimeEnabled=Sensor,
 ] interface Sensor : EventTarget {
     readonly attribute SensorState state;
     readonly attribute SensorReading? reading;
 
     [CallWith=ScriptState, RaisesException] void start();
     [CallWith=ScriptState, RaisesException] void stop();
 
     attribute EventHandler onerror;
     attribute EventHandler onchange;
-    attribute EventHandler onstatechange;
+    attribute EventHandler onactivate;
 };