feat: introduce puppeteer-firefox (#3628)

This adds a proof-of-concept of `puppeteer-firefox`.
This consists of two parts:
- `//experimental/juggler` - patches to apply to Firefox.
- `//experimental/puppeteer-firefox` - front-end code to
be merged with Puppeteer.

As things become more stable, we'll gradually move it out of
the experimental folder.

experimental/puppeteer-firefox/test/firefoxonly.spec.js

@@ -0,0 +1,24 @@
+module.exports.addTests = function({testRunner, expect, product}) {
+  const {describe, xdescribe, fdescribe} = testRunner;
+  const {it, fit, xit} = testRunner;
+  const {beforeAll, beforeEach, afterAll, afterEach} = testRunner;
+
+  describe('Firefox-specific tests', function() {
+    describe('Browser.version', function() {
+      it('should return whether we are in headless', async({browser}) => {
+        const version = await browser.version();
+        expect(version.length).toBeGreaterThan(0);
+        expect(version.startsWith('Firefox/')).toBe(true);
+      });
+    });
+
+    describe('Browser.userAgent', function() {
+      it('should include WebKit', async({browser}) => {
+        const userAgent = await browser.userAgent();
+        expect(userAgent.length).toBeGreaterThan(0);
+        expect(userAgent).toContain('Gecko');
+      });
+    });
+  });
+};
+