Patch XHR-based document leak

On device testing, we saw some document leaks that were traced back
to XHR-related closures in Cordova. This tries to clean up our handling
of the xhr variable by ensuring it's nulled out after use to avoid
circular refs.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)