base

Increase the stability of sorting in the Model.

This is necessary to prevent UI thrash (items continually shuffling
around) when the underlying Provider doesn't return its contents in a
stable order from one load event to another (DownloadStorageProvider is
an example).

BUG=26417297

Change-Id: Ie99e56b610f5d01d5318be07c0379b506c828735