Have MimeSniffingResourceHandler call WillStartRequest on new Handlers.

content/browser/loader/intercepting_resource_handler.h

Index: content/browser/loader/intercepting_resource_handler.h
diff --git a/content/browser/loader/intercepting_resource_handler.h b/content/browser/loader/intercepting_resource_handler.h
index 8729e5b271ed7a72efb8f1f202f7d1a011f4041c..4be72b85d466db7ed83224b507eb1c169d53d778 100644
--- a/content/browser/loader/intercepting_resource_handler.h
+++ b/content/browser/loader/intercepting_resource_handler.h
@@ -42,14 +42,16 @@ class CONTENT_EXPORT InterceptingResourceHandler
   // 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: