fix(firefox): enable domains in a proper order (#4077)

Otherwise we might get console messages from Page domain before
we get execution contexts reported.

experimental/puppeteer-firefox/lib/Page.js

@@ -27,9 +27,9 @@ class Page extends EventEmitter {
   static async create(session, target, defaultViewport) {
     const page = new Page(session, target);
     await Promise.all([
-      session.send('Page.enable'),
-      session.send('Network.enable'),
       session.send('Runtime.enable'),
+      session.send('Network.enable'),
+      session.send('Page.enable'),
     ]);
 
     if (defaultViewport)
@@ -729,9 +729,6 @@ class Page extends EventEmitter {
     return await this._frameManager.mainFrame().setContent(html);
   }
 
-  _onClosed() {
-  }
-
   _onConsole({type, args, executionContextId, location}) {
     const context = this._frameManager.executionContextById(executionContextId);
     this.emit(Events.Page.Console, new ConsoleMessage(type, args.map(arg => createHandle(context, arg)), location));