Disable modal JavaScript dialogs for platform apps.

chrome/test/data/extensions/platform_apps/modal_dialogs/manifest.json

Index: chrome/test/data/extensions/platform_apps/modal_dialogs/manifest.json
diff --git a/chrome/test/data/extensions/platform_apps/modal_dialogs/manifest.json b/chrome/test/data/extensions/platform_apps/modal_dialogs/manifest.json
new file mode 100644
index 0000000000000000000000000000000000000000..e85ae2b801b7f69c587ecc74eee257be51fefdd9
--- /dev/null
+++ b/chrome/test/data/extensions/platform_apps/modal_dialogs/manifest.json
@@ -0,0 +1,13 @@
+{
+  "name": "Platform App Test: modal dialogs are disallowed",
+  "platform_app": true,
+  "version": "1",
+  "manifest_version": 2,
+  "background": { "scripts": ["background.js"] },
+  "app": {
+    "launch": {
+      "local_path": "main.html",
+      "container": "shell"
+    }
+  }
+}