[Sensors] Align sensor reading attribute implementation with the specification

This patch updates sensor reading implementation accordingly to specification [1] which says: "the reading attribute of two instances of the same Sensor interface associated with the same sensor hold the same SensorReading during a single event loop turn".

Besides, this patch also drops 'SensorReadingEvent' and 'SensorReadingEventInit' types which no longer exist in the specification.

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

BUG=606766

Review-Url: https://codereview.chromium.org/2472403002
Cr-Commit-Position: refs/heads/master@{#430578}
22 files changed