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/HTMLMenuItemElement.cpp

@@ -24,8 +24,8 @@ namespace dom {
 #define NS_MENUITEM_TYPE(bits) ((bits) & ~( \
   NS_CHECKED_IS_TOGGLED | NS_ORIGINAL_CHECKED_VALUE))
 
-enum CmdType                                                                 
-{                                                                            
+enum CmdType : uint8_t
+{
   CMD_TYPE_MENUITEM = 1,
   CMD_TYPE_CHECKBOX,
   CMD_TYPE_RADIO
@@ -35,7 +35,7 @@ static const nsAttrValue::EnumTable kMenuItemTypeTable[] = {
   { "menuitem", CMD_TYPE_MENUITEM },
   { "checkbox", CMD_TYPE_CHECKBOX },
   { "radio", CMD_TYPE_RADIO },
-  { 0 }
+  { nullptr, 0 }
 };
 
 static const nsAttrValue::EnumTable* kMenuItemDefaultType =