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

content/browser/loader/url_loader_request_handler.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_LOADER_URL_LOADER_REQUEST_HANDLER_H_
 #define CONTENT_BROWSER_LOADER_URL_LOADER_REQUEST_HANDLER_H_
 
 #include <memory>
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "content/public/common/url_loader.mojom.h"
 #include "content/public/common/url_loader_factory.mojom.h"
 
 namespace content {
 
 class ResourceContext;
 struct ResourceRequest;
 
 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.
 class CONTENT_EXPORT URLLoaderRequestHandler {
  public:
   URLLoaderRequestHandler() = default;
   virtual ~URLLoaderRequestHandler() = default;
 
   // Calls |callback| with a non-null StartLoaderCallback if this handler
   // can handle the request, calls it with null callback otherwise.
   virtual void MaybeCreateLoader(const ResourceRequest& resource_request,
                                  ResourceContext* resource_context,
                                  LoaderCallback callback) = 0;
 
   // Returns the URLLoaderFactory if any to be used for subsequent URL requests
   // going forward. Subclasses who want to handle subresource requests etc may
   // want to override this to return a custom factory.
   virtual mojom::URLLoaderFactoryPtr MaybeCreateSubresourceFactory();
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_LOADER_URL_LOADER_REQUEST_HANDLER_H_