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/HTMLHRElement.cpp

@@ -43,7 +43,7 @@ HTMLHRElement::ParseAttribute(int32_t aNamespaceID,
     { "left", NS_STYLE_TEXT_ALIGN_LEFT },
     { "right", NS_STYLE_TEXT_ALIGN_RIGHT },
     { "center", NS_STYLE_TEXT_ALIGN_CENTER },
-    { 0 }
+    { nullptr, 0 }
   };
 
   if (aNamespaceID == kNameSpaceID_None) {