Adding `content_security_policy` to a few sample extensions.

chrome/common/extensions/docs/examples/api/notifications/style.css

Index: chrome/common/extensions/docs/examples/api/notifications/style.css
diff --git a/chrome/common/extensions/docs/examples/api/notifications/style.css b/chrome/common/extensions/docs/examples/api/notifications/style.css
new file mode 100644
index 0000000000000000000000000000000000000000..7a5de73d526d18df87afbadecd52edac8229176e
--- /dev/null
+++ b/chrome/common/extensions/docs/examples/api/notifications/style.css
@@ -0,0 +1,25 @@
+/**
+ * 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.
+ */
+
+/* Clone the look and feel of "chrome://" pages. */
+body {
+  margin: 10px;
+  font: 84% Arial, sans-serif
+}
+
+h1 { font-size: 156% }
+
+h1 img {
+  margin: 1px 5px 0 1px;
+  vertical-align: middle
+}
+
+h2 {
+  border-top: 1px solid #9cc2ef;
+  background-color: #ebeff9;
+  padding: 3px 5px;
+  font-size: 100%
+}