Continue key propagation for extensions without any listeners to chrome.commands.onCommand.

chrome/test/data/extensions/api_test/keybinding/continue_propagation/injected.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var port = chrome.extension.connect({name: 'continue_propagation'});
+document.body.addEventListener('keyup', function(evt) {
+  if (evt.keyCode == 70 /* F */)
+    port.postMessage({result: true});
+  }, true);