When InterceptingResourceHandler swaps in a handler, call OnWillStart.

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 937b8d7237df1f9e4525c76a09ea04e96a674848..02856e3f0457d2a51a1b93401d116af2b546b612 100644
--- a/content/browser/loader/intercepting_resource_handler.h
+++ b/content/browser/loader/intercepting_resource_handler.h
@@ -81,11 +81,17 @@ class CONTENT_EXPORT InterceptingResourceHandler
     // Resume().
     SENDING_PAYLOAD_TO_OLD_HANDLER,
 
-    // The InterceptingResourcHandler is notifying OnResponseStarted to the new
-    // handler and waiting for its completion via Resume(). After the
-    // completion, the InterceptingResourcHandler will transit to
+    // The InterceptingResourcHandler is calling the new handler's
+    // OnResponseStarted method and waiting for its completion via Resume().
+    // After the completion, the InterceptingResourceHandler will transition to
+    // SENDING_ON_RESPONSE_STARTED_TO_NEW_HANDLER on success.
+    SENDING_ON_WILL_START_TO_NEW_HANDLER,
+
+    // The InterceptingResourcHandler is calling the new handler's
+    // OnResponseStarted method and waiting for its completion via Resume().
+    // After the completion, the InterceptingResourceHandler will transition to
     // WAITING_FOR_ON_READ_COMPLETED on success.
-    NOTIFYING_ON_RESPONSE_STARTED_TO_NEW_HANDLER,
+    SENDING_ON_RESPONSE_STARTED_TO_NEW_HANDLER,
 
     // The InterceptingResourcHandler is waiting for OnReadCompleted to be
     // called.
@@ -111,6 +117,7 @@ class CONTENT_EXPORT InterceptingResourceHandler
   bool SendPayloadToOldHandler(bool* defer);
   bool SendFirstReadBufferToNewHandler(bool* defer);
   bool SendCompletionToOldHandler(bool* defer);
+  bool SendOnResponseStartedToNewHandler(bool* defer);
 
   State state_ = State::STARTING;