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/HTMLBRElement.cpp

@@ -34,7 +34,7 @@ static const nsAttrValue::EnumTable kClearTable[] = {
   { "right", NS_STYLE_CLEAR_RIGHT },
   { "all", NS_STYLE_CLEAR_BOTH },
   { "both", NS_STYLE_CLEAR_BOTH },
-  { 0 }
+  { nullptr, 0 }
 };
 
 bool