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This protocol allows the wayland server to notify the client of
connected gamepads and their state.
The protocol does not support axis mapping but assumes the use of the
'Standard Gamepad' as defined by the W3C gamepad API.

BUG=620977

Review-Url: https://codereview.chromium.org/2093803002
Cr-Commit-Position: refs/heads/master@{#405238}