[Favicon] Add new fallback icon rendering flow.

chrome/browser/ui/webui/favicon_source.h

Index: chrome/browser/ui/webui/favicon_source.h
diff --git a/chrome/browser/ui/webui/favicon_source.h b/chrome/browser/ui/webui/favicon_source.h
index e4a8fe9503601cd7b2d59f53c41704da3aabb752..e66dabfe4d2e46865fdb7c8a2dc265217ebc9035 100644
--- a/chrome/browser/ui/webui/favicon_source.h
+++ b/chrome/browser/ui/webui/favicon_source.h
@@ -21,7 +21,7 @@ class Profile;
 // requests for favicons and the history backend that serves these.
 //
 // Format:
-//   chrome://favicon/size&scalefactor/urlmodifier/url
+//   chrome://favicon/size&scalefactor/fallback&specs/urlmodifier/url
 // Some parameters are optional as described below. However, the order of the
 // parameters is not interchangeable.
 //
@@ -41,7 +41,16 @@ class Profile;
 //      If the parameter is unspecified, the requested favicon's size defaults
 //      to 16 and the requested scale factor defaults to 1x.
 //      Example: chrome://favicon/size/16@2x/http://www.google.com/
-//  'urlmodifier'      Optional
+//  'fallback&specs'    Optional
+//    Values: ['fallback/<specs>/']
+//    'fallback/specs/':
+//      Specifies explicit request of a fallback favicon. <specs> is a comma-
+//      separated string of fallback specifications. For details, see
+//      /components/favicon_base/fallback_icon_specs_builder.h.
+//     Example: chrome://favicon/fallback/fff,000/http://www.google.com/
+//      This requests the fallback favicon for http://www.google.com, using
+//      #fff as the background color and #000 as the text color.
+//  'urlmodifier'       Optional
 //    Values: ['iconurl', 'origin']
 //    'iconurl': Specifies that the url parameter refers to the URL of
 //    the favicon image as opposed to the URL of the page that the favicon is