Storage clearing

Mozilla have written some tests for storage that depend on clean storage 
for each test. As a result I have a review request to add a method (in 
wptrunner) to clear the storage. It's currently looking for URLs 
starting /storage/, which doesn't seem terribly principled.

Do people think that adding <meta name=storage content=clear> to these 
tests would be a reasonable idea? Is there a better approach? If we add 
that metadata how implementable is it? Gecko can use marionette to run 
chrome-context code. I imagine that implementing it in wptrunner for 
Chrome would involve restarting the browser to get a clean profile. I"m 
not sure about Safari/Edge (maybe one could just delete some files on 
disk?).