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/HTMLFormElement.cpp

@@ -87,7 +87,7 @@ static const uint8_t NS_FORM_AUTOCOMPLETE_OFF = 0;
 static const nsAttrValue::EnumTable kFormAutocompleteTable[] = {
   { "on",  NS_FORM_AUTOCOMPLETE_ON },
   { "off", NS_FORM_AUTOCOMPLETE_OFF },
-  { 0 }
+  { nullptr, 0 }
 };
 // Default autocomplete value is 'on'.
 static const nsAttrValue::EnumTable* kFormDefaultAutocomplete = &kFormAutocompleteTable[0];