chore: refactor Browser.js into seperate files (#2097)

This patch splits Browser.js into multiple separate files.

lib/TaskQueue.js

@@ -0,0 +1,17 @@
+class TaskQueue {
+  constructor() {
+    this._chain = Promise.resolve();
+  }
+
+  /**
+   * @param {function()} task
+   * @return {!Promise}
+   */
+  postTask(task) {
+    const result = this._chain.then(task);
+    this._chain = result.catch(() => {});
+    return result;
+  }
+}
+
+module.exports = TaskQueue;