Introduce background.scripts feature for extension manifests.

chrome/test/data/extensions/api_test/background_scripts/b.js

+// Copyright (c) 2012 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.
+
+// We're just testing that multiple scripts get added to a page in-order
+// and are run.
+chrome.test.assertEq("hi!", test);
+
+// Also test the injection point is consistent. We want to inject into body
+// because having a document.body element is convenient for some scripts.
+var scripts = document.querySelectorAll("script");
+chrome.test.assertEq(2, scripts.length);
+for (var i = 0, script; script = scripts[i]; i++) {
+  chrome.test.assertEq("BODY", script.parentElement.nodeName);
+}
+
+chrome.test.notifyPass();