[Sensors] Implement sensor data polling

This patch contains implementations of SensorPollingStrategy interface, defining when sensor reading is updated from shared buffer.

Also it contains logic for Suspend/Resume mojo API calls based on the page visibility. For this purpose 'SensorProxy' inherits 'PageVisibilityObserver'.

Specification: https://w3c.github.io/sensors

BUG=606766

Review-Url: https://codereview.chromium.org/2323683002
Cr-Commit-Position: refs/heads/master@{#417541}
5 files changed