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

Source/modules/app_banner/AppBannerController.h

Index: Source/modules/app_banner/AppBannerController.h
diff --git a/Source/modules/app_banner/AppBannerController.h b/Source/modules/app_banner/AppBannerController.h
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+++ b/Source/modules/app_banner/AppBannerController.h
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+// 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 AppBannerController_h
+#define AppBannerController_h
+
+namespace blink {
+
+enum class WebAppBannerPromptReply;
+class LocalFrame;
+class WebString;
+
+// FIXME: unless userChoice ends up implemented, this class should not exist and
+// a regular static method could be used instead.
+class AppBannerController final {
+public:
+    static void willShowInstallBannerPrompt(LocalFrame*, const WebString& platform, WebAppBannerPromptReply*);
+
+private:
+    AppBannerController() = delete;
+    ~AppBannerController() = delete;
+};
+
+} // namespace blink
+
+#endif // AppBannerController_h