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

ui/base/webui/web_ui_util.h

Index: ui/base/webui/web_ui_util.h
diff --git a/ui/base/webui/web_ui_util.h b/ui/base/webui/web_ui_util.h
index 84e06f3170be5d3e336a932458efc5d5447f4f25..2dbcfc72e008376f87470ebe63143a70677a7213 100644
--- a/ui/base/webui/web_ui_util.h
+++ b/ui/base/webui/web_ui_util.h
@@ -47,9 +47,20 @@ UI_BASE_EXPORT void ParsePathAndScale(const GURL& url,
 
 // Helper function to set the font family, size, and text direction into the
 // given dictionary.
+// Note that using font family and size in JS templates is deprecated, because
+// it might lead to flicker. Instead, use AppendWebUICSSTextDefaults() below.
+// TODO(bauerb): Remove uses of font-family and font-size in JS templates, then
+// rename this method to SetTextDirection().
 UI_BASE_EXPORT void SetFontAndTextDirection(
     base::DictionaryValue* localized_strings);
 
+// Get a CSS declaration for common text styles for all of Web UI.
+UI_BASE_EXPORT std::string GetWebUiCssTextDefaults();
+
+// Appends the CSS declaration returned by GetWebUiCssTextDefaults() as an
+// inline stylesheet.
+UI_BASE_EXPORT void AppendWebUiCssTextDefaults(std::string* html);
+
 // Get some common font styles for all of WebUI.
 UI_BASE_EXPORT std::string GetFontFamily();
 UI_BASE_EXPORT std::string GetFontSize();