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/HTMLTableCaptionElement.cpp

@@ -32,7 +32,7 @@ static const nsAttrValue::EnumTable kCaptionAlignTable[] = {
   { "right",  NS_STYLE_CAPTION_SIDE_RIGHT },
   { "top",    NS_STYLE_CAPTION_SIDE_TOP },
   { "bottom", NS_STYLE_CAPTION_SIDE_BOTTOM },
-  { 0 }
+  { nullptr,  0 }
 };
 
 bool