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This patch implements AmbientLightSensor [1] blink bindings and adds
LayoutTest helpers for testing sensors that are based on Generic Sensor API.

Following layout tests added to test new functionality:
IDL tests.
 - third_party/WebKit/LayoutTests/sensor/idl-AmbientLightSensor.html
 - third_party/WebKit/LayoutTests/sensor/idl-AmbientLightSensorReading.html
AmbientLightSensor tests.
 - third_party/WebKit/LayoutTests/sensor/ambient-light-sensor.html

Intent to Implement:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/TkfdVqYAYiE/xLGN2b1-AAAJ

[1] ED specification for Ambient Light Sensor http://w3c.github.io/ambient-light/

BUG=606766

Review-Url: https://codereview.chromium.org/2332323002
Cr-Commit-Position: refs/heads/master@{#419438}