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

Source/modules/app_banner/BeforeInstallPromptEvent.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/app_banner/BeforeInstallPromptEvent.h"
 
 #include "bindings/core/v8/CallbackPromiseAdapter.h"
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
@@ skipping 31 matching lines @@
 {
 }
 
 Vector<String> BeforeInstallPromptEvent::platforms() const
 {
     return m_platforms;
 }
 
 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
 {
     return EventNames::BeforeInstallPromptEvent;
 }
 
 ScriptPromise BeforeInstallPromptEvent::prompt(ScriptState* scriptState)
 {
     if (m_client && defaultPrevented() && !m_redispatched) {
         ASSERT(m_requestId != -1);
         m_redispatched = true;
         m_client->showAppBanner(m_requestId);
         return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
     }
 
     return ScriptPromise::rejectWithDOMException(scriptState, DOMException::create(InvalidStateError, "The prompt() method may only be called once, following preventDefault()."));
 }
 
 } // namespace blink