Enable nsAttrValue::EnumTable to be initialized with enum. So, we could get rid
of the castings in EnumTable. Fix EnumTable initialization comment.
For those untyped enumerations, declare them with uint8_t, as to other typed
enumerations with type size larger than int16_t, force casting to int16_t.
Use {nullptr,0} instead of {0} to represent the last entry.
MozReview-Commit-ID: 7Dma3Apkmxj
