feat(defaultTheme): fallback userAgent for platform string

Since `navigator.platform` is deprecated and could be removed in the
future, fallback to `navigator.userAgent` if prev one does not exists.

See also:
https://developer.mozilla.org/en-US/docs/Web/API/Navigator/platform

Author: marjune163

src/tpl/defaultTheme/frontend/index.js

@@ -347,7 +347,7 @@
 
 		var SKIP_TAGS = ['INPUT', 'BUTTON', 'TEXTAREA'];
 
-		var PLATFORM = navigator.platform;
+		var PLATFORM = navigator.platform || navigator.userAgent;
 		var IS_MAC_PLATFORM = PLATFORM.indexOf('Mac') >= 0 || PLATFORM.indexOf('iPhone') >= 0 || PLATFORM.indexOf('iPad') >= 0 || PLATFORM.indexOf('iPod') >= 0
 
 		var lookupKey;