[w3c/manifest] purpose member type check missing (#771)

When processing purpose, the spec assumes that the value will be a String. However, it could be any other JSON type. 

@dominickng or @mgiuca, could you kindly please check what Chrome does when given:

```JSON
{
    icons: [{"src": "test", "sizes": "400x400", "purpose": [] }]
}
``` 

Does it convert purpose to "any"? or does the icon get ignored (failure)? 

Also, probably too late... but `purpose` should have been a List :( When we do canonicalization, we return it as a list. Anyway, no big deal. 


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