Add sample extension that allows setting plugin-specific content settings.

chrome/common/extensions/docs/examples/extensions/plugin_settings/domui/css/button.css

Index: chrome/common/extensions/docs/examples/extensions/plugin_settings/domui/css/button.css
diff --git a/chrome/common/extensions/docs/examples/extensions/plugin_settings/domui/css/button.css b/chrome/common/extensions/docs/examples/extensions/plugin_settings/domui/css/button.css
new file mode 100644
index 0000000000000000000000000000000000000000..5564f426d8c9915f8419a8d2170708b120010228
--- /dev/null
+++ b/chrome/common/extensions/docs/examples/extensions/plugin_settings/domui/css/button.css
@@ -0,0 +1,43 @@
+button,
+input[type='button'],
+input[type='submit'] {
+  -webkit-border-radius: 2px;
+  -webkit-box-shadow: 0px 1px 3px rgba(0, 0, 0, 0.1);
+  -webkit-user-select: none;
+  background: -webkit-linear-gradient(#fafafa, #f4f4f4 40%, #e5e5e5);
+  border: 1px solid #aaa;
+  color: #444;
+  font-size: inherit;
+  margin-bottom: 0px;
+  min-width: 4em;
+  padding: 3px 12px 3px 12px;
+}
+
+button:hover,
+input[type='button']:hover,
+input[type='submit']:hover {
+  -webkit-box-shadow: 0px 1px 3px rgba(0, 0, 0, 0.2);
+  background: #ebebeb -webkit-linear-gradient(#fefefe, #f8f8f8 40%, #e9e9e9);
+  border-color: #999;
+  color: #222;
+}
+
+button:active,
+input[type='button']:active,
+input[type='submit']:active {
+  -webkit-box-shadow: inset 0px 1px 3px rgba(0, 0, 0, 0.2);
+  background: #ebebeb -webkit-linear-gradient(#f4f4f4, #efefef 40%, #dcdcdc);
+  color: #333;
+}
+
+button[disabled],
+input[type='button'][disabled],
+input[type='submit'][disabled],
+button[disabled]:hover,
+input[type='button'][disabled]:hover,
+input[type='submit'][disabled]:hover {
+  -webkit-box-shadow: none;
+  background: -webkit-linear-gradient(#fafafa, #f4f4f4 40%, #e5e5e5);
+  border-color: #aaa;
+  color: #888;
+}