IWYU: add missing include for std::unique_ptr

Change-Id: I70cedfa4d20b09192ba749a0866ac531aeaf0db2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601365Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842841}

ui/webui/webui_config.h

@@ -5,6 +5,7 @@
 #ifndef UI_WEBUI_WEBUI_CONFIG_H_
 #define UI_WEBUI_WEBUI_CONFIG_H_
 
+#include <memory>
 #include <string>
 
 #include "base/strings/string_piece.h"