Add hooks for BeforeInstallPromptEvent to be redispatched.

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 3a7aac581fe5f812821d1d266184e72aa56fa4dc..ad9a23268d40c4b7f9fcb30484802325c3c3645f 100644
--- a/Source/modules/app_banner/BeforeInstallPromptEvent.h
+++ b/Source/modules/app_banner/BeforeInstallPromptEvent.h
@@ -40,6 +40,10 @@ public:
 
     virtual const AtomicString& interfaceName() const override;
 
+    // This is called after the site has first called preventDefault(), and now
+    // wishes to redispatch the event at a later point in their own context.

[mlamouri (slow - plz ping), 2015/06/06 21:40:55]

I don't think this comment is useful. prompt() is directly linked to the
bindings. It's merely boilerplate at that point.

+    void prompt();
+
 private:
     BeforeInstallPromptEvent();
     BeforeInstallPromptEvent(const AtomicString& name, const Vector<String>& platforms, int requestId, WebAppBannerClient*);
@@ -50,6 +54,7 @@ private:
 
     int m_requestId;
     WebAppBannerClient* m_client;
+    bool m_redispatched;
 };
 
 DEFINE_TYPE_CASTS(BeforeInstallPromptEvent, Event, event, event->interfaceName() == EventNames::BeforeInstallPromptEvent, event.interfaceName() == EventNames::BeforeInstallPromptEvent);