Refactor BeforeInstallPromptEvent to use ScriptPromiseProperty

LayoutTests/app_banner/app-banner-event-prompt.html

Index: LayoutTests/app_banner/app-banner-event-prompt.html
diff --git a/LayoutTests/app_banner/app-banner-event-prompt.html b/LayoutTests/app_banner/app-banner-event-prompt.html
index b5edae164c381abd3475a70c2f4f61f631bfeee5..f328d6af1343dd8cefe3f6cc938c2cb1eabd973c 100644
--- a/LayoutTests/app_banner/app-banner-event-prompt.html
+++ b/LayoutTests/app_banner/app-banner-event-prompt.html
@@ -92,13 +92,16 @@ function prompt_test(index) {
             assert_equals(e.platforms[0], 'foo', 'First platform');
             assert_equals(e.platforms[1], 'bar', 'Second platform');
 
+            if (test_case.cancel) {
+                e.preventDefault();
+            }
+
             if (test_case.late) {
                 return;
             }
 
             if (test_case.cancel) {
-                // Cancel the event, then call prompt() to restart the display pipeline.
-                e.preventDefault();
+                // Call prompt() to restart the pipeline.
                 verify_prompt_resolve(e, t);
             } else {
                 // A call to prompt() before preventDefault should reject.
@@ -118,7 +121,6 @@ function prompt_test(index) {
         // Test that firing prompt() outside of the handler resolves or rejects correctly.
         if (test_case.late) {
             if (test_case.cancel) {
-                event.preventDefault();
                 verify_prompt_resolve(event, t);
             } else {
                 verify_prompt_reject(event, t);