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This CL extends the device::GamepadConsumer,
device::mojom::GamepadConsumer, and blink::WebGamepadListener interfaces
to allow gamepad button and axis events to be communicated from //device
to blink. These will be used to implement gamepad button and axis
events in javascript.

BUG=856290

Change-Id: I8ce83d5e4225e79c3b25c7d7ef65684e9ada404f
Reviewed-on: https://chromium-review.googlesource.com/1161492Reviewed-by: Ken Buchanan <kenrb@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@{#581421}