[Sensors] Remove SensorReading interfaces

The SensorReading interface has been recently excluded from W3C Generic Sensor specification [1].
Now concrete sensor interfaces should themselves contain reading attributes.

Sensor layout and browser tests are updated accordingly.

BUG=606766

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

Review-Url: https://codereview.chromium.org/2668173003
Cr-Commit-Position: refs/heads/master@{#447749}
52 files changed