Add hooks for BeforeInstallPromptEvent to be redispatched.

public/platform/modules/app_banner/WebAppBannerClient.h

Index: public/platform/modules/app_banner/WebAppBannerClient.h
diff --git a/public/platform/modules/app_banner/WebAppBannerClient.h b/public/platform/modules/app_banner/WebAppBannerClient.h
index 57164c15b52ef7079d3f1c98746afcef22eb2e86..ea4a01eb06644b8247f0b017186f98cb6fdd7838 100644
--- a/public/platform/modules/app_banner/WebAppBannerClient.h
+++ b/public/platform/modules/app_banner/WebAppBannerClient.h
@@ -18,6 +18,9 @@ public:
 
     // 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