Gamepad API: tests for gamepad events

This CL adds a basic test for 'gamepadconnected' and 'gamepaddisconnected' events and extends the api test to check the differences between the prefixed and non-prefixed api's.

BUG=344556

Review URL: https://codereview.chromium.org/212813008

git-svn-id: svn://svn.chromium.org/blink/trunk@170461 bbb929c8-8fbe-4397-9dbb-9b2b20218538