Have MimeSniffingResourceHandler call WillStartRequest on new Handlers.

content/browser/loader/intercepting_resource_handler.h

 // Copyright 2016 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_INTERCEPTING_RESOURCE_HANDLER_H_
 #define CONTENT_BROWSER_LOADER_INTERCEPTING_RESOURCE_HANDLER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 24 matching lines @@
   // ResourceHandler implementation:
   bool OnResponseStarted(ResourceResponse* response, bool* defer) override;
   bool OnWillRead(scoped_refptr<net::IOBuffer>* buf,
                   int* buf_size,
                   int min_size) override;
   bool OnReadCompleted(int bytes_read, bool* defer) override;
 
   // Replaces the next handler with |new_handler|, sending
   // |payload_for_old_handler| to the old handler. Must be called after
   // OnWillStart and OnRequestRedirected and before OnResponseStarted. One
-  // OnWillRead call is permitted beforehand. |new_handler|'s OnWillStart and
-  // OnRequestRedirected methods will not be called.
+  // OnWillRead call is permitted beforehand. |new_handler| is assumed to
+  // be advanced to a state where it can take the place of the old handler
+  // (The InterceptingResourceHandler won't call OnWillStart or
+  // OnRequestRedirected, but will pass along newly read data).
   void UseNewHandler(std::unique_ptr<ResourceHandler> new_handler,
                      const std::string& payload_for_old_handler);
 
   // Used in tests.
-  ResourceHandler* new_handler_for_testing() const {
-    return new_handler_.get();
+  ResourceHandler* next_handler_for_testing() const {
+    return next_handler_.get();
   }
 
  private:
   enum class State {
     // In this state, the InterceptingResourceHandler is waiting for the mime
     // type of the response to be identified, to check if the next handler
     // should be replaced with an appropriate one.
     STARTING,
 
     // In this state, the InterceptingResourceHandler is waiting to copy the
@@ skipping 23 matching lines @@
   scoped_refptr<net::IOBuffer> first_read_buffer_;
   size_t first_read_buffer_size_;
   scoped_refptr<net::IOBuffer> first_read_buffer_copy_;
 
   DISALLOW_COPY_AND_ASSIGN(InterceptingResourceHandler);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_LOADER_INTERCEPTING_RESOURCE_HANDLER_H_