Directly inline shared WebUI CSS declarations into the security and supervised user interstitials.

ui/webui/resources/css/text_defaults.css

Index: ui/webui/resources/css/text_defaults.css
diff --git a/ui/webui/resources/css/text_defaults.css b/ui/webui/resources/css/text_defaults.css
index 3e9007a00a0510ecf6d841ee66f10032e6fade80..682a858526bc4fad6881b4e1765e08298d733576 100644
--- a/ui/webui/resources/css/text_defaults.css
+++ b/ui/webui/resources/css/text_defaults.css
@@ -6,11 +6,14 @@
  * some per-platform/locale styles that dramatically alter the page. This is
  * done to reduce flicker, as JS may not run before the page is rendered.
  *
- * Include this stylesheet via its chrome://resources/ URL, i.e.:
+ * There are two ways to include this stylesheet:
+ * 1. via its chrome://resources/ URL in HTML, i.e.:
  *
- *   <link rel="stylesheet" href="chrome://resources/css/font_defaults.css">
+ *   <link rel="stylesheet" href="chrome://resources/css/text_defaults.css">
  *
- * otherwise its $placeholders wont be expanded. */
+ * 2. via the webui::AppendWebUICSSTextDefaults() method to directly append it
+ * to an HTML string.
+ * Otherwise its $placeholders won't be expanded. */
 
 html {
   direction: $1;