Make eventInitDict param optional in GamepadEvent constructor

[nondebug]

Closes #216

The following tasks have been completed:

• Modified Web platform tests (link to pull request)

Implementation commitment:

• WebKit
• Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
• Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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Preview | Diff

[tcl3]

Should the type of the gamepad attribute also be nullable, since it can now return null?

[marcoscaceres]

@tcl3, yes, good catch. I think the attributes are required to mirror the value they are initialized with.

[weinig]

Other than matching current implementations, is there a reason/use case for a null gamepad on GamepadEvent. In WebKit at least, the engine will never create a GamepadEvent with a null Gamepad (other than in this case where the user constructs it from JS). Is there reason to think there is a compat issue with fixing engines to require it?