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Most gamepads report their button state in a HID output report that
associates HID usages with the state of each button. For most gamepads,
the HID usages associated with buttons come from the Button
usage page.

For some gamepads, particularly those designed to work on Android
devices, the Back and Home buttons are associated with usages outside
the normal set of gamepad usages. For instance, the Xbox One S
gamepad, when connected over Bluetooth, sends a SystemMainMenu usage
for the Home button. The same gamepad, after receiving a firmware
update to improve Android compatibility, reports the Home and Back
buttons as Home and Back usages on the Consumer usage page.

On Linux, we use the joydev input driver to enumerate connected gamepads
and fetch the current gamepad state. However, HID buttons outside the
normal set of gamepad usages are not reported through joydev and must
be read through evdev instead.

This CL allows these extra buttons to be detected and mapped by Chrome.

BUG=824871

Change-Id: Ib873948c6632edb36675f45c948190cb99ce126e
Reviewed-on: https://chromium-review.googlesource.com/1091464Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569439}