The IDL for getInstalledRelatedApps:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/installedapp/NavigatorInstalledApp.idl

    Promise<RelatedApplication> getInstalledRelatedApps();

This should be:

    Promise<sequence<RelatedApplication>> getInstalledRelatedApps();

The implementation does already return an array object. It's just a type error in the IDL. I believe the bindings generator ignores the Promise resolution type, which is why this type error isn't detected by the compiler.