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

Source/modules/app_banner/AppBannerController.cpp

Index: Source/modules/app_banner/AppBannerController.cpp
diff --git a/Source/modules/app_banner/AppBannerController.cpp b/Source/modules/app_banner/AppBannerController.cpp
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index 0000000000000000000000000000000000000000..6a99bbce937175015fe4d45a2af0d6531d238fee
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+++ b/Source/modules/app_banner/AppBannerController.cpp
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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.
+
+#include "config.h"
+#include "modules/app_banner/AppBannerController.h"
+
+#include "core/EventTypeNames.h"
+#include "core/frame/DOMWindow.h"
+#include "core/frame/LocalFrame.h"
+#include "modules/app_banner/BeforeInstallPromptEvent.h"
+#include "platform/RuntimeEnabledFeatures.h"
+
+namespace blink {
+
+// static
+void AppBannerController::willShowInstallBannerPrompt(LocalFrame* frame, const WebString& platform, bool* cancel)
+{
+    ASSERT(RuntimeEnabledFeatures::appBannerEnabled());
+
+    // dispatchEvent() returns whether the default behavior can happen. In other
+    // words, it returns false if preventDefault() was called.
+    *cancel = !frame->domWindow()->dispatchEvent(BeforeInstallPromptEvent::create(EventTypeNames::beforeinstallprompt, platform));
+}
+
+} // namespace blink