Fix a memory leak in the app banner prompt layout tests.

Source/modules/app_banner/BeforeInstallPromptEvent.h

Index: Source/modules/app_banner/BeforeInstallPromptEvent.h
diff --git a/Source/modules/app_banner/BeforeInstallPromptEvent.h b/Source/modules/app_banner/BeforeInstallPromptEvent.h
index c5fd15324ee23665d52d795555b05280cd3a8757..7c6ff6427e9a8472a6cf8ad30cb5640b32928bdb 100644
--- a/Source/modules/app_banner/BeforeInstallPromptEvent.h
+++ b/Source/modules/app_banner/BeforeInstallPromptEvent.h
@@ -6,6 +6,7 @@
 #define BeforeInstallPromptEvent_h
 
 #include "bindings/core/v8/ScriptPromise.h"
+#include "bindings/core/v8/ScriptPromiseResolver.h"
 #include "modules/EventModules.h"
 #include "platform/heap/Handle.h"
 
@@ -48,7 +49,7 @@ private:
     BeforeInstallPromptEvent(const AtomicString& name, const BeforeInstallPromptEventInit&);
 
     Vector<String> m_platforms;
-    ScriptPromise m_userChoice;
+    RefPtrWillBeMember<ScriptPromiseResolver> m_resolver;
 
     int m_requestId;
     WebAppBannerClient* m_client;