I’m a bit surprised Chrome developers went the route of an in memory filesystem instead of trying to sandbox and clear real disk access. Silently using up to 120MB without realizing sounds pretty bad.
Maybe they could encrypt with a key kept in memory? That'd still allow detection of use though.
Might seem like an incidental concern, but being able to vacuum up a pattern of incognito sessions from a seized laptop (at a border crossing, say) and correlate it with the activity of an online pseudonym could be pretty useful.
Just check periodically (at startup?) for orphaned temporary storage data. I'm sure there are other parts of the browser that need to do this sort of thing anyway - expired cache data, for example.
EDIT: I suppose these are often backed by memory anyway, so not sure if this would solve the problem, but interested in hearing arguments around it nonetheless.
A browser requires a generic solution to prevent denial of service due to excessive resource consumption.