eventInitDict parameter in GamepadEvent constructor should be optional

[nondebug]

The GamepadEvent constructor is defined with two required parameters:

constructor(DOMString type, GamepadEventInit eventInitDict);

This differs from the signature in implementations which define the second parameter as optional (blink, Gecko, WebKit):

  constructor(DOMString type, optional GamepadEventInit eventInitDict = {}); // blink
  constructor(DOMString type, optional GamepadEventInit eventInitDict = {});  // Gecko
  constructor([AtomString] DOMString type, optional GamepadEventInit eventInitDict);  // WebKit