Use native `String.prototype.trim` in `String.prototype.trim` when available. (kangax)

src/lang/string.js

@@ -407,7 +407,9 @@ Object.extend(String.prototype, (function() {
     sub:            sub,
     scan:           scan,
     truncate:       truncate,
-    strip:          strip,
+    // Firefox 3.5+ supports String.prototype.trim
+    // (`trim` is ~ 5x faster than `strip` in FF3.5)
+    strip:          String.prototype.trim ? String.prototype.trim : strip,
     stripTags:      stripTags,
     stripScripts:   stripScripts,
     extractScripts: extractScripts,