I intend to turn Clear-Site-Data header on by default in 63. The last
remaining dependency bug is going to land today; we pass all the WPTs.
It has been developed behind the dom.clearSiteData.enabled preference.
Chrome has this feature enabled since 06-2017.
Bug to turn on by default: bug 1470111.
Spec:
https://w3c.github.io/webappsec-clear-site-data/