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/HTMLButtonElement.cpp

@@ -47,7 +47,7 @@ static const nsAttrValue::EnumTable kButtonTypeTable[] = {
   { "button", NS_FORM_BUTTON_BUTTON },
   { "reset", NS_FORM_BUTTON_RESET },
   { "submit", NS_FORM_BUTTON_SUBMIT },
-  { 0 }
+  { nullptr, 0 }
 };
 
 // Default type is 'submit'.