Fix a memory leak in the app banner prompt layout tests.

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..6fed8f7732e7561a74d68c7c4c8f3a9bcdc3d187 100644
--- a/LayoutTests/app_banner/app-banner-event-prompt.html
+++ b/LayoutTests/app_banner/app-banner-event-prompt.html
@@ -44,7 +44,7 @@ var prompt_test_cases = [
 ];
 
 function verify_prompt_resolve(e, t) {
-    e.prompt().then(function() { }, t.unreached_func("prompt() promise should resolve."));
+    e.prompt().then(undefined, t.unreached_func("prompt() promise should resolve."));
 }
 
 function verify_prompt_reject(e, t) {
@@ -75,30 +75,34 @@ function verify_userChoice(e, t, test_case, index) {
     }), t.unreached_func("userChoice promise should resolve."));
 }
 
+var events = []; // For storing the beforeinstallprompt event.
 function prompt_test(index) {
     if (index >= prompt_test_cases.length)
         return;
 
     var test_case = prompt_test_cases[index];
     async_test(function(t) {
-        var event = null;
         var event_handler = t.step_func(function(e) {
             // Remove the event handler to prevent it being used in subsequent
             // invocations of prompt_test(). Save event object for call outside handler.
             window.removeEventListener('beforeinstallprompt', event_handler);
-            event = e;
 
             assert_equals(e.platforms.length, 2, 'Number of platforms');
             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) {
+                // Exit the handler once we've stopped the event.
+                events.push(e);
                 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.
@@ -117,14 +121,14 @@ function prompt_test(index) {
 
         // Test that firing prompt() outside of the handler resolves or rejects correctly.
         if (test_case.late) {
+            e = events.pop();
             if (test_case.cancel) {
-                event.preventDefault();
-                verify_prompt_resolve(event, t);
+                verify_prompt_resolve(e, t);
             } else {
-                verify_prompt_reject(event, t);
+                verify_prompt_reject(e, t);
             }
             // Check userChoice and call the next test.
-            verify_userChoice(event, t, test_case, index);
+            verify_userChoice(e, t, test_case, index);
         }
     }, test_case.name);
 }