Updating Blink-side Gamepad API to match latest spec

BUG=344556
R=abarth@chromium.org, scottmg@chromium.org

Review URL: https://codereview.chromium.org/170993002

git-svn-id: svn://svn.chromium.org/blink/trunk@167539 bbb929c8-8fbe-4397-9dbb-9b2b20218538