Note that you'll only be able to use a separate StoragePartition for WebContents objects that are in different browsing context groups (i.e., BrowsingInstances), since same-origin documents in two tabs that can script each other must stay in the same process and thus StoragePartition. (window.open() is one example of creating a second WebContents in the same browsing context group.)
Still, it should be possible to modify things to use a different StoragePartition for most independently created tabs. rmcelrath@ and alexmos@ were just working on a refactor that might make that easier, and could probably share context.
Charlie