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On ChromeOS the accelerometer needs to be read by both ash/ and content/

This change removes the ownership of AccelerometerReader from Shell, changing it into a Singleton.

TEST=Confirmed that a device with an accelerometer enters maximize mode.
BUG=342908

Review URL: https://codereview.chromium.org/822503003

Cr-Commit-Position: refs/heads/master@{#311353}