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This CL defines the GamepadButtonEvent and GamepadAxisEvent types that
will be used to return button and axis events to event listeners.

BUG=856290

Change-Id: I52d4eb91f9e40fe86e45d498e8aa97ec341341d6
Reviewed-on: https://chromium-review.googlesource.com/c/1167976Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605863}