Safe Browsing V4 gradual roll-out

https://bugzilla.mozilla.org/show_bug.cgi?id=1393980

Building the add-on

zip -r ../sbv4-gradual-rollout.xpi bootstrap.js install.rdf

Installing the add-on in Firefox

1. about:config, set
   1. extensions.legacy.enabled to true
   2. xpinstall.signatures.required to false
2. about:debugging > [load temporary addon] > choose dist/addon.xpi`
3. tools > Web Developer > Browser Toolbox