[sensors] Add a browser test to sanity check ambient light sensor.

The idea of the test is to sanity check the entire generic sensor
integration especially to verify that the data put in the shared
buffer is correctly handled by Blink.

The patch add a way to provide a fake provider which will create
fake sensors so we have a reliable and hardware independent solution.

The rest of the testing is happening either as layout tests or
as unittests in device/generic_sensor.

BUG=606766

Review-Url: https://codereview.chromium.org/2434563005
Cr-Commit-Position: refs/heads/master@{#428025}
10 files changed