WPT: Create function to test BFCache not restored reason

IndexedDB/back-forward-cache-open-connection.window.js

@@ -26,6 +26,9 @@ promise_test(async t => {
   // Create an IndexedDB database with higher version.
   await createIndexedDBForTesting(rc2, 'test_idb_2', 2);
   await rc2.historyBack();
-  // The previous page receiving versionchange event should be evicted.
-  await assert_not_bfcached(rc1);
+  // The previous page receiving versionchange event should be evicted with the
+  // correct reason.
+  // `kIgnoreEventAndEvict` will be reported as "Internal error".
+  // See `NotRestoredReasonToReportString()`.
+  await assert_not_bfcached(rc1, ['Internal error']);
 });

html/browsers/browsing-the-web/back-forward-cache/resources/rc-helper.js

@@ -25,12 +25,62 @@ async function assert_implements_bfcache(remoteContextHelper) {
   assert_implements_optional(beforeBFCache == true, 'BFCache not supported.');
 }
 
-// If the value in window is undefined, this means that the page was reloaded,
-// i.e., the page was not restored from BFCache.
+// A helper function to assert that the page is not restored from BFCache by
+// checking whether the `beforeBFCache` value from `window` is undefined
+// due to page reload.
+// This function also takes an optional `notRestoredReasons` list which
+// indicates the set of expected reasons that make the page not restored.
+// If the reasons list is undefined, the check will be skipped. Otherwise
+// this check will use the `notRestoredReasons` API, to obtain the reasons
+// in a tree structure, and flatten the reasons before making the order-
+// insensitive comparison.
+// If the API is not available, the function will terminate instead of marking
+// the assertion failed.
 // Call `prepareForBFCache()` before navigating away to call this function.
-async function assert_not_bfcached(remoteContextHelper) {
+async function assert_not_bfcached(
+    remoteContextHelper, notRestoredReasons) {
   var beforeBFCache = await getBeforeBFCache(remoteContextHelper);
   assert_equals(beforeBFCache, undefined);
+
+  // The reason is optional, so skip the remaining test if the
+  // `notRestoredReasons` is not set.
+  if (notRestoredReasons === undefined) {
+    return;
+  }
+
+  let isFeatureEnabled = await remoteContextHelper.executeScript(() => {
+    return 'notRestoredReasons' in performance.getEntriesByType('navigation')[0];
+  });
+
+  // Return if the `notRestoredReasons` API is not available.
+  if (!isFeatureEnabled) {
+    return;
+  }
+
+  let result = await remoteContextHelper.executeScript(() => {
+    return performance.getEntriesByType('navigation')[0].notRestoredReasons;
+  });
+
+  let expectedNotRestoredReasonsSet = new Set(notRestoredReasons);
+  let notRestoredReasonsSet = new Set();
+
+  // Flatten the reasons from the main frame and all the child frames.
+  const collectReason = (node) => {
+    for (let reason of node.reasons) {
+      notRestoredReasonsSet.add(reason);
+    }
+    for (let child of node.children) {
+      collectReason(child);
+    }
+  }
+  collectReason(result);
+
+  assert_equals(notRestoredReasonsSet.length,
+      expectedNotRestoredReasonsSet.length);
+
+  for (let reason of expectedNotRestoredReasonsSet) {
+    assert_true(notRestoredReasonsSet.has(reason));
+  }
 }
 
 // A helper function that combines the steps of setting window property,