Bug 1297306 - part5:create enum constructors for EnumTable. r=baku

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

dom/html/HTMLTrackElement.cpp

@@ -67,7 +67,7 @@ static constexpr nsAttrValue::EnumTable kKindTable[] = {
   { "descriptions", static_cast<int16_t>(TextTrackKind::Descriptions) },
   { "chapters", static_cast<int16_t>(TextTrackKind::Chapters) },
   { "metadata", static_cast<int16_t>(TextTrackKind::Metadata) },
-  { 0 }
+  { nullptr, 0 }
 };
 
 // Invalid values are treated as "metadata" in ParseAttribute, but if no value