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

Source/modules/app_banner/BeforeInstallPromptEventInit.idl

Index: Source/modules/app_banner/BeforeInstallPromptEventInit.idl
diff --git a/Source/core/html/canvas/WebGLSync.idl b/Source/modules/app_banner/BeforeInstallPromptEventInit.idl
similarity index 65%
copy from Source/core/html/canvas/WebGLSync.idl
copy to Source/modules/app_banner/BeforeInstallPromptEventInit.idl
index a401e20aa857447b84d5b6878d452cafca642fe5..5fac0385bbc7f9eb5d4ec14dee29c984c248d400 100644
--- a/Source/core/html/canvas/WebGLSync.idl
+++ b/Source/modules/app_banner/BeforeInstallPromptEventInit.idl
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-[
-    RuntimeEnabled=UnsafeES3APIs,
-    WillBeGarbageCollected
-] interface WebGLSync {
-};
+dictionary BeforeInstallPromptEventInit : EventInit {
+    DOMString platform = "web";
+};