Suppress "non-existent property" warnings in spf.url

Tighter property checking in JsCompiler is warning about the `username`
and `password` property access on `a` elements.  Temporarily suppress
these warnings until the default externs can be fixed or the property
accesses are removed.

src/client/url/url.js

@@ -7,6 +7,7 @@
  * @fileoverview URL manipulation functions.
  *
  * @author [email protected] (Alex Nicksay)
+ * @suppress {missingProperties}
  */
 
 goog.provide('spf.url');