[Sensors] Implement Sensor.onactivate

In the updated Generic Sensor specification [1] 'onactivate' event handler replaces 'onstatechange' event handler.

[1] https://w3c.github.io/sensors/#sensor-onactivate

BUG=606766

Review-Url: https://codereview.chromium.org/2481363003
Cr-Commit-Position: refs/heads/master@{#431546}
5 files changed