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In order to avoid re-entrancy issues, this CL modifies sensor event
dispatching code and uses postTask to schedule event dispatch.
New test is added to ambient-light-sensor.html that checks if generic
sensor code works correctly when sensor is not supported

BUG=606766

Review-Url: https://codereview.chromium.org/2381913002
Cr-Commit-Position: refs/heads/master@{#422761}