Add support for fallback content for the frame. This includes main and subframes.

content/browser/loader/url_loader_request_handler.h

Index: content/browser/loader/url_loader_request_handler.h
diff --git a/content/browser/loader/url_loader_request_handler.h b/content/browser/loader/url_loader_request_handler.h
index fb0a1f4b2f31e521926888dc0f9ac9090e4e72a9..f639b6f11ab0311d703049b27e3061ad8c66bec6 100644
--- a/content/browser/loader/url_loader_request_handler.h
+++ b/content/browser/loader/url_loader_request_handler.h
@@ -20,7 +20,14 @@ using StartLoaderCallback =
     base::OnceCallback<void(mojom::URLLoaderRequest request,
                             mojom::URLLoaderClientPtr client)>;
 
-using LoaderCallback = base::OnceCallback<void(StartLoaderCallback)>;
+// Provided by handlers who provide fallback handling.
+using ResponseFallback =
+    base::OnceCallback<bool(const ResourceResponseHead& response,
+                            mojom::URLLoaderClientPtr client,
+                            mojom::URLLoaderRequest request)>;
+
+using LoaderCallback =
+    base::OnceCallback<void(StartLoaderCallback, ResponseFallback)>;
 
 // An instance of this class is a per-request object and kept around during
 // the lifetime of a request (including multiple redirect legs) on IO thread.