Editorial: Fix Link Warnings

[alcooper91]

We needed a no-op change to force a recreation of the Editor's working draft, (Issue #77); so I decided to fix a few link warnings.

• Add Gamepad spec reference to fix missing GamepadMappingType link
• convert void->undefined, as per changes to WebIDL standard webidl (whatwg/webidl#906)
• convert [=context object=]'s {{XR}} object's -> {{Navigator/xr}}'s, which appears to be the link intended
• Update SetViews algorithm to directly check the secondaryViews array, rather than an undefined variable.