What is this suggestion about?
Currently, selecting Develop → Empty Cookies and Storage for *website* provides no feedback to the user, which could lead to confusion about whether it was actually clicked. Similarly, there should ideally be an Are you sure? prompt, as is the case with Develop → Empty Cache.

While no output is the desired outcome with many tools, the browser should not be one of those. In particular, since we are approaching the 1.0 release, ideally with user-friendly communication when it comes to the user interface.
On that note, either of the following should be enough:
Option 1: Add an are you sure? prompt before cookies and storage are cleared
Pros: This would work similarly to other options such as Empty Cache, it would prevent accidentally clearing cookies/storage.
Option 2: Add a message about the cookies and storage being cleared. This could also include information about how much space was freed up, etc.
Personally, I think Option 1 would be the better choice, but other users might prefer Option 2.