"Browser-Specific" APIs don't show the WebKit legacy

The [metrics doc](https://docs.google.com/document/d/1gSyOBsedPuxlNsoPM3N4hXPqOViLVX0jtrw9Dmsq7Ck/edit) defines browser-specific as:

> How many APIs does this engine ship that appear in fewer than 2 other engines for at least a year?

`README.md`, `confluence.html`, and the metric calculator (`lib/confluence/browser_specific.es6.js`) document and implement this as:

> How many APIs does this engine ship that appear in *no other engines* for at least a year?

This is important to fix especially because the *common WebKit lineage* of Safari and Chrome deflate their metric.