Make GamepadPollingInterval configurable by Finch

Add a GamepadPollingInterval feature with a variation parameter
to allow the gamepad polling interval to be overridden by a
server change.

BUG=855188

Change-Id: I16e9fa9957d64b09f00e0c7d1ce5e762799332f3
Reviewed-on: https://chromium-review.googlesource.com/1130004
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575318}
7 files changed