Test DeviceMotionEventPump to ensure events deliver at 60Hz

      Add a unit test for DeviceMotionEventPump to ensure that events are not delivered to Blink at a rate faster than 60 Hz.

TEST=DeviceMotionEventPumpTest.PumpThrottlesEventRate
BUG=421691

Review URL: https://codereview.chromium.org/870053009

Cr-Commit-Position: refs/heads/master@{#314574}