Make this.loading abortable

.then() removes the .abort() method from the promise, even though
we had return jqxhr; in the then() callback. So store xhr in
a variable and put the abort method back.

This fixes a JS error upon calling this.loading.abort();

Change-Id: I50460782e58399198bacc02d028984682ddbed56
Merge "Make this.loading abortable"