Have MimeSniffingResourceHandler call WillStartRequest on new Handlers.

content/browser/loader/mime_sniffing_resource_handler.h

Index: content/browser/loader/mime_sniffing_resource_handler.h
diff --git a/content/browser/loader/mime_sniffing_resource_handler.h b/content/browser/loader/mime_sniffing_resource_handler.h
index 05b901a9c0cb51ca16509e4bc63b6bcf14d8ffca..bb3e3fdb566467710b8101cb11199fe0ab4e79b8 100644
--- a/content/browser/loader/mime_sniffing_resource_handler.h
+++ b/content/browser/loader/mime_sniffing_resource_handler.h
@@ -64,13 +64,17 @@ class CONTENT_EXPORT MimeSniffingResourceHandler
 
     // In this state, the MimeSniffingResourceHandler has identified the mime
     // type and made a decision on whether the request should be intercepted or
-    // not. It is nows attempting to replay the response to downstream
-    // handlers.
-    STATE_INTERCEPTION_CHECK_DONE,
+    // not.  It now needs to prepare the new handler for use, and replay the
+    // already received data.
+    STATE_NEED_TO_PREPARE_TO_USE_NEW_HANDLER,
+
+    // In this state, the MimeSniffingResourceHandler is preparing the new
+    // handler for use.
+    STATE_PREPARING_TO_USE_NEW_HANDLER,
 
     // In this state, the MimeSniffingResourceHandler is replaying the buffered
     // OnResponseStarted event to the downstream ResourceHandlers.
-    STATE_REPLAYING_RESPONSE_RECEIVED,
+    STATE_REPLAYING_RECEIVED_RESPONSE,
 
     // In this state, the MimeSniffingResourceHandler is just a blind
     // pass-through
@@ -108,8 +112,12 @@ class CONTENT_EXPORT MimeSniffingResourceHandler
   // Intercepts the request as a stream/download if needed.
   bool MaybeIntercept(bool* defer);
 
+  // Prepares the new handler for use, advancing it to a state where it can
+  // be swapped in to replace the InterceptingResourceHandler's old one.
+  bool PrepareToUseNewHandler(bool* defer);
+
   // Replays OnResponseStarted on the downstream handlers.
-  bool ReplayResponseReceived(bool* defer);
+  bool ReplayReceivedResponse(bool* defer);
 
   // Replays OnReadCompleted on the downstreams handlers.
   bool ReplayReadCompleted(bool* defer);
@@ -169,6 +177,9 @@ class CONTENT_EXPORT MimeSniffingResourceHandler
 
   RequestContextType request_context_type_;
 
+  std::unique_ptr<ResourceHandler> new_handler_;
+  std::string payload_for_old_handler_;
+
   base::WeakPtrFactory<MimeSniffingResourceHandler> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MimeSniffingResourceHandler);