[w3c/manifest] Related applications parsing is incorrect (#936)

The [steps for processing the `related_applications` member](https://www.w3.org/TR/appmanifest/#dfn-processing-the-related_applications-member) has the following issues:

1. It references a `src` value which is not specified.
2. Even though `platform` is required, it does not throw out `ExternalApplicationResource`s that do not include a `platform`

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/936