Fix a crash in MimeHandlerServiceImpl::AbortStream().

extensions/browser/api/mime_handler_private/mime_handler_private.h

Index: extensions/browser/api/mime_handler_private/mime_handler_private.h
diff --git a/extensions/browser/api/mime_handler_private/mime_handler_private.h b/extensions/browser/api/mime_handler_private/mime_handler_private.h
index 8a067dd799dd5e4284fffc954f0eaa5e5c2b2fe5..045264a18a79d9a79efd4579efe0cce4b48892cb 100644
--- a/extensions/browser/api/mime_handler_private/mime_handler_private.h
+++ b/extensions/browser/api/mime_handler_private/mime_handler_private.h
@@ -10,6 +10,7 @@
 
 namespace extensions {
 class StreamContainer;
+class MimeHandlerServiceImplTest;
 
 class MimeHandlerServiceImpl
     : public mojo::InterfaceImpl<mime_handler::MimeHandlerService> {
@@ -19,6 +20,8 @@ class MimeHandlerServiceImpl
       mojo::InterfaceRequest<mime_handler::MimeHandlerService> request);
 
  private:
+  friend class MimeHandlerServiceImplTest;
+
   explicit MimeHandlerServiceImpl(
       base::WeakPtr<StreamContainer> stream_container);
   ~MimeHandlerServiceImpl() override;