[Sensors] Stop exposing sensor state

In the updated specification [1] sensor states are internal slots, the 'state' attribute is moved to the 'boolean activated' attribute.
This CL updates the Sensor implementation and layout tests accordingly.

[1] https://w3c.github.io/sensors/

BUG=606766

Review-Url: https://codereview.chromium.org/2798913002
Cr-Commit-Position: refs/heads/master@{#462434}
13 files changed