security settings panel: clear browsing data now button

[bradleyrichter]

Browsing data can be cleared immediately (vs automatically on browser close) by using the "Clear Browsing Data Now..." button.

A modal (to the page) dialog opens and provides the same switches for a batch clearing action.

The button action can be canceled if desired. (for users who are only exploring the functionality)

[cndouglas]

Could this option also be added to the hamburger menu for easy access? Similar to Firefox's Forget button.

[bbondy]

@liunkae pls post a separate issue for that, thanks.