Prerender: Outcome test on OnTriggerDestroyed for pending prerender

This CL adds the browser test that confirms that when activating the
host that is ready for activation, the other pending prerender hosts outcome can be recorded as kRunning.

We may expect kTriggeredButPending (this is not supported yet) as
outcome in this case, but this CL creates the test for checking which
outcome the current implementation records just for now.

For more information, please see "Integration with Precog API" section
in the following design doc:
https://docs.google.com/document/d/1kLZOTzuRwwb-CBQKryxzu5GUZdFFX-MOV7PncdFefxY/edit?usp=sharing

Bug: 1355151
Change-Id: I57aaf69293172b01b47beaa7e12aec04af251126
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3925083
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Yoshiki Tanioka <tanioka@google.com>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1054076}

content/browser/preloading/prerender/prerender_browsertest.cc

@@ -4384,6 +4384,87 @@ IN_PROC_BROWSER_TEST_F(PrerenderSequentialPrerenderingBrowserTest,
   }
 }
 
+// Test that when the running prerender is destroyed due to the activation of
+// another already prerendered page, some pending prerender's outcome is
+// recorded as `kRunning` because the running prerender's cancellation brings
+// the pending prerender's initial navigation before the pending prerender is
+// cancelled.
+// TODO(crbug.com/1355151): Record kTriggeredButPending instead in
+// this case once the outcome value is newly supported.
+IN_PROC_BROWSER_TEST_F(
+    PrerenderSequentialPrerenderingBrowserTest,
+    PreloadingTriggeringOutcomeForStartingPrerenderBeforeDestruction) {
+  net::test_server::ControllableHttpResponse response2(
+      embedded_test_server(), "/empty.html?prerender2");
+  ASSERT_TRUE(embedded_test_server()->Start());
+  const GURL kInitialUrl = embedded_test_server()->GetURL("/empty.html");
+  const GURL kPrerender1 =
+      embedded_test_server()->GetURL("/empty.html?prerender1");
+  const GURL kPrerender2 =
+      embedded_test_server()->GetURL("/empty.html?prerender2");
+  const GURL kPrerender3 =
+      embedded_test_server()->GetURL("/empty.html?prerender3");
+
+  ASSERT_TRUE(NavigateToURL(shell(), kInitialUrl));
+
+  test::PrerenderHostRegistryObserver registry_observer(*web_contents_impl());
+
+  // Insert 3 URLs into the speculation rules at the same time.
+  AddMultiplePrerenderAsync({kPrerender1, kPrerender2, kPrerender3});
+  registry_observer.WaitForTrigger(kPrerender3);
+  test::PrerenderHostObserver prerender1_observer(*web_contents(),
+                                                  GetHostForUrl(kPrerender1));
+
+  // Stop the second prerendering initial navigation.
+  response2.WaitForRequest();
+
+  NavigationHandleObserver activation_observer(web_contents(), kPrerender1);
+
+  // Activate prerender1. The trigger should destroy all the other prerender
+  // hosts.
+  NavigatePrimaryPage(kPrerender1);
+  prerender1_observer.WaitForActivation();
+  EXPECT_EQ(web_contents()->GetLastCommittedURL(), kPrerender1);
+  EXPECT_TRUE(prerender1_observer.was_activated());
+
+  {
+    ukm::SourceId ukm_source_id = activation_observer.next_page_ukm_source_id();
+
+    auto ukm_entries = test_ukm_recorder()->GetEntries(
+        Preloading_Attempt::kEntryName, test::kPreloadingAttemptUkmMetrics);
+    EXPECT_EQ(ukm_entries.size(), 3u);
+
+    std::vector<UkmEntry> expected_entries = {
+        attempt_ukm_entry_builder().BuildEntry(
+            ukm_source_id, PreloadingType::kPrerender,
+            PreloadingEligibility::kEligible,
+            PreloadingHoldbackStatus::kAllowed,
+            PreloadingTriggeringOutcome::kSuccess,
+            PreloadingFailureReason::kUnspecified,
+            /*accurate=*/true),
+        attempt_ukm_entry_builder().BuildEntry(
+            ukm_source_id, PreloadingType::kPrerender,
+            PreloadingEligibility::kEligible,
+            PreloadingHoldbackStatus::kAllowed,
+            PreloadingTriggeringOutcome::kRunning,
+            PreloadingFailureReason::kUnspecified,
+            /*accurate=*/false),
+        attempt_ukm_entry_builder().BuildEntry(
+            ukm_source_id, PreloadingType::kPrerender,
+            PreloadingEligibility::kEligible,
+            PreloadingHoldbackStatus::kAllowed,
+            PreloadingTriggeringOutcome::kRunning,
+            PreloadingFailureReason::kUnspecified,
+            /*accurate=*/false),
+    };
+
+    EXPECT_THAT(ukm_entries,
+                testing::UnorderedElementsAreArray(expected_entries))
+        << content::test::ActualVsExpectedUkmEntriesToString(ukm_entries,
+                                                             expected_entries);
+  }
+}
+
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
                        IsInactiveAndDisallowActivationCancelsPrerendering) {
   const GURL kInitialUrl = GetUrl("/empty.html");