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/HTMLMediaElement.cpp

@@ -3221,7 +3221,7 @@ bool HTMLMediaElement::ParseAttribute(int32_t aNamespaceID,
     { "none",     HTMLMediaElement::PRELOAD_ATTR_NONE },
     { "metadata", HTMLMediaElement::PRELOAD_ATTR_METADATA },
     { "auto",     HTMLMediaElement::PRELOAD_ATTR_AUTO },
-    { 0 }
+    { nullptr,    0 }
   };
 
   if (aNamespaceID == kNameSpaceID_None) {