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

Source/modules/app_banner/BeforeInstallPromptEvent.cpp

Index: Source/modules/app_banner/BeforeInstallPromptEvent.cpp
diff --git a/Source/modules/app_banner/BeforeInstallPromptEvent.cpp b/Source/modules/app_banner/BeforeInstallPromptEvent.cpp
index 74cdab48b06f844afaf94119b8a34e03fb48a2cc..b7259f7acf04892c9b3b3cb391c953a6ad3fe5d3 100644
--- a/Source/modules/app_banner/BeforeInstallPromptEvent.cpp
+++ b/Source/modules/app_banner/BeforeInstallPromptEvent.cpp
@@ -49,14 +49,15 @@ Vector<String> BeforeInstallPromptEvent::platforms() const
 
 ScriptPromise BeforeInstallPromptEvent::userChoice(ScriptState* scriptState)
 {
-    if (m_userChoice.isEmpty() && m_client) {
-        ASSERT(m_requestId != -1);
-        RefPtrWillBeRawPtr<ScriptPromiseResolver> resolver = ScriptPromiseResolver::create(scriptState);
-        m_userChoice = resolver->promise();
-        m_client->registerBannerCallbacks(m_requestId, new CallbackPromiseAdapter<AppBannerPromptResult, void>(resolver));
+    if (!m_resolver) {
+        m_resolver = ScriptPromiseResolver::create(scriptState);
+        if (m_client) {
+            ASSERT(m_requestId != -1);
+            m_client->registerBannerCallbacks(m_requestId, new CallbackPromiseAdapter<AppBannerPromptResult, void>(m_resolver));
+        }
     }
 
-    return m_userChoice;
+    return m_resolver->promise();
 }
 
 const AtomicString& BeforeInstallPromptEvent::interfaceName() const