browser(firefox): support BrowserContext.evaluateOnNewDocument

[dgozman]

dgozman/gecko-dev@70ee31b

[jperl]

This is great, now we do not have to poll for pages to inject our scripts. We could eliminate our page polling entirely if you created a similar API for exposeFunction:

await browserContext.exposeFunction('myFunction', (..args, page) => {
  // can use the page here
});

Or emit a "pagecreated" event from the BrowserContext that works for all pages, not just popups.