[AppBanner] Blink implementation of an event when an install banner is shown.

Source/modules/app_banner/BeforeInstallPromptEvent.idl

Index: Source/modules/app_banner/BeforeInstallPromptEvent.idl
diff --git a/Source/modules/app_banner/BeforeInstallPromptEvent.idl b/Source/modules/app_banner/BeforeInstallPromptEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..1444c7e5716d1f70a68568e772dd2d3686e16816
--- /dev/null
+++ b/Source/modules/app_banner/BeforeInstallPromptEvent.idl
@@ -0,0 +1,11 @@
+// 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.
+
+[
+    Constructor(DOMString platform, optional BeforeInstallPromptEventInit eventInitDict),
+    RuntimeEnabled=AppBanner,
+] interface BeforeInstallPromptEvent : Event {
+    readonly attribute DOMString platform;
+    [CallWith=ScriptState] readonly attribute Promise<DOMString> userChoice;
+};