Add GamepadMappingType enum at gamepad.idl

According to spec[1], type of Gamepad.mapping attribute
should be GamepadMappingType which has two values("" and "standard")

[1] https://www.w3.org/TR/gamepad/#gamepadmappingtype-enum

Bug: 954409
Change-Id: Id9944f31e5a43f2a2f89e602678e003554429056
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575048
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654273}
1 file changed