Don't use ES6 Number.isNaN

Number.isNaN is a new function introduced in ECMAScript 6.
MediaWiki only requires ECMAScript 5 supports from browsers.
Notably, Opera 12 does not have Number.isNaN. Instead, use
the global isNaN function (which behaves the same except for
non-numeric inputs).

Change-Id: If436cd26b21ce0336dfbc37144f6226e7b948e5e

src/userSettings.js

@@ -83,7 +83,7 @@ export default function createUserSettings( storage ) {
 			count = parseInt( result, 10 );
 
 			// stored number is not a zero, override it to zero and store new value
-			if ( Number.isNaN( count ) ) {
+			if ( isNaN( count ) ) {
 				count = 0;
 				this.setPreviewCount( count );
 			}