Add hooks for BeforeInstallPromptEvent to be redispatched.

public/platform/modules/app_banner/WebAppBannerClient.h

 // 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.
 
 #ifndef WebAppBannerClient_h
 #define WebAppBannerClient_h
 
 #include "public/platform/WebCallbacks.h"
 #include "public/platform/modules/app_banner/WebAppBannerPromptResult.h"
 
 namespace blink {
 
 using WebAppBannerCallbacks = WebCallbacks<WebAppBannerPromptResult, void>;
 
 class WebAppBannerClient {
 public:
     virtual ~WebAppBannerClient() { }
 
     // Ownership of the callbacks is transferred to the client.
     virtual void registerBannerCallbacks(int requestId, WebAppBannerCallbacks*) = 0;
+
+    // Tell the client that the event is being redispatched.
+    virtual void showAppBanner(int requestId) { }
 };
 
 } // namespace blink
 
 #endif // WebAppBannerClient_h