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/utils.js

@@ -0,0 +1,98 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+const fs = require('fs');
+const path = require('path');
+
+const utils = module.exports = {
+  /**
+   * @param {!Page} page
+   * @param {string} frameId
+   * @param {string} url
+   * @return {!Puppeteer.Frame}
+   */
+  attachFrame: async function(page, frameId, url) {
+    await page.evaluate(attachFrame, frameId, url);
+
+    async function attachFrame(frameId, url) {
+      const frame = document.createElement('iframe');
+      frame.src = url;
+      frame.id = frameId;
+      document.body.appendChild(frame);
+      await new Promise(x => frame.onload = x);
+    }
+  },
+
+  /**
+   * @param {!Page} page
+   * @param {string} frameId
+   */
+  detachFrame: async function(page, frameId) {
+    await page.evaluate(detachFrame, frameId);
+
+    function detachFrame(frameId) {
+      const frame = document.getElementById(frameId);
+      frame.remove();
+    }
+  },
+
+  /**
+   * @param {!Page} page
+   * @param {string} frameId
+   * @param {string} url
+   */
+  navigateFrame: async function(page, frameId, url) {
+    await page.evaluate(navigateFrame, frameId, url);
+
+    function navigateFrame(frameId, url) {
+      const frame = document.getElementById(frameId);
+      frame.src = url;
+      return new Promise(x => frame.onload = x);
+    }
+  },
+
+  /**
+   * @param {!Frame} frame
+   * @param {string=} indentation
+   * @return {!Array<string>}
+   */
+  dumpFrames: function(frame, indentation) {
+    indentation = indentation || '';
+    let description = frame.url().replace(/:\d{4}\//, ':<PORT>/');
+    if (frame.name())
+      description += ' (' + frame.name() + ')';
+    let result = [indentation + description];
+    for (const child of frame.childFrames())
+      result.push(...utils.dumpFrames(child, '    ' + indentation));
+    return result;
+  },
+
+  /**
+   * @param {!EventEmitter} emitter
+   * @param {string} eventName
+   * @return {!Promise<!Object>}
+   */
+  waitEvent: function(emitter, eventName, predicate = () => true) {
+    return new Promise(fulfill => {
+      emitter.on(eventName, function listener(event) {
+        if (!predicate(event))
+          return;
+        emitter.removeListener(eventName, listener);
+        fulfill(event);
+      });
+    });
+  },
+};