We should record the percentage data savings at the start of each browsing session. This should be straightforward to write, modulo two considerations:

First, we should only record savings for users who have browsed a reasonably amount. For example, we could limit this to users that have fetched at least 10MB of page data, or have navigated to at least 10 pages, or who have had Data Saver enabled for at least 10 days. I think I have a slight preference for the constraints in that order.

Second, we should do something special to record negative savings. That could be a second histogram, and the first histogram would report 0% for any session that had negative savings. Note crbug.com/812910 is related.