Adding `content_security_policy` to proxy sample extension.

chrome/common/extensions/docs/examples/extensions/proxy_configuration/background.js

Index: chrome/common/extensions/docs/examples/extensions/proxy_configuration/background.js
diff --git a/chrome/common/extensions/docs/examples/extensions/proxy_configuration/background.js b/chrome/common/extensions/docs/examples/extensions/proxy_configuration/background.js
new file mode 100644
index 0000000000000000000000000000000000000000..8640d419e0d16678da4f4af6155a92c79bf2365f
--- /dev/null
+++ b/chrome/common/extensions/docs/examples/extensions/proxy_configuration/background.js
@@ -0,0 +1,23 @@
+// Copyright (c) 2011 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.
+
+/**
+ * @fileoverview This file initializes the background page by loading a
+ * ProxyErrorHandler, and resetting proxy settings if required.
+ *
+ * @author Mike West <mkwst@google.com>
+ */
+
+document.addEventListener("DOMContentLoaded", function () {
+  var errorHandler = new ProxyErrorHandler();
+
+  // If this extension has already set the proxy settings, then reset it
+  // once as the background page initializes.  This is essential, as
+  // incognito settings are wiped on restart.
+  var persistedSettings = ProxyFormController.getPersistedSettings();
+  if (persistedSettings !== null) {
+    chrome.proxy.settings.set(
+        {'value': persistedSettings.regular});
+  }
+});