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This CL adds a command line switch and Feature entry for enabling
event listeners to listen for gamepad button and axis inputs.

Intent to implement thread:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/9cfRxXx5Cis

BUG=856290

Change-Id: I5323ff7692ffb235fc3759b1305216085ed6460c
Reviewed-on: https://chromium-review.googlesource.com/1139092
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580361}