[Ext SW] Fix installation rejection in FoundRegistrationForStartWorker.

ServiceWorkerContextWrapper::StartWorkerForScope searches for
registration with active or installing worker through
DidFindRegistrationForFindImpl. For the case where there is an
installingworker, DidFindRegistrationForFindImpl can see a
registration.installing_version(), but the version can be gone by
the time it PostTask-s to FoundRegistrationForStartWorker
if the service worker script rejected the 'install' event.

This CL removes the DCHECK that incorrectly assumes either active
or installing version *must* be present
in FoundRegistrationForStartWorker. The DCHECK failure is evident
in the flaky extension test failure:
ServiceWorkerBasedBackgroundTest.WorkerStartFailureClearsPendingTasks.
The CL also enables the mentioned test.

Bug: 1063476
Change-Id: I12dd3b5f2155bc8b8677f9734443832c1a96fe61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119208Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753555}

chrome/browser/extensions/service_worker_apitest.cc

@@ -1946,11 +1946,8 @@ IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest,
 // PendingTasks correctly. Also tests that subsequent tasks are properly
 // cleared.
 // Regression test for https://crbug.com/1019161.
-//
-// Flaky (https://crbug.com/1063476): Fails DCHECK(version_ptr) in
-// ServiceWorkerContextWrapper.
 IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest,
-                       DISABLED_WorkerStartFailureClearsPendingTasks) {
+                       WorkerStartFailureClearsPendingTasks) {
   content::StoragePartition* storage_partition =
       content::BrowserContext::GetDefaultStoragePartition(browser()->profile());
   content::ServiceWorkerContext* context =
@@ -1990,14 +1987,14 @@ IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest,
   ASSERT_EQ(kTestExtensionId, extension->id());
   LazyContextId context_id(browser()->profile(), extension->id(),
                            extension->url());
-  ServiceWorkerStartFailureObserver worker_start_failure_observer(
-      extension->id());
-
   // Let the worker start so it rejects 'install' event. This causes the worker
   // to stop.
   observer.WaitForWorkerStart();
   observer.WaitForWorkerStop();
 
+  ServiceWorkerStartFailureObserver worker_start_failure_observer(
+      extension->id());
+
   ServiceWorkerTaskQueue* service_worker_task_queue =
       ServiceWorkerTaskQueue::Get(browser()->profile());
   // Adding a pending task to ServiceWorkerTaskQueue will try to start the

content/browser/service_worker/service_worker_context_wrapper.cc

@@ -126,12 +126,17 @@ void FoundRegistrationForStartWorker(
                                           ? registration->active_version()
                                           : registration->installing_version();
   // Since FindRegistrationForScope returned
-  // blink::ServiceWorkerStatusCode::kOk, there must be either: -
-  // an active version, which optionally might have activated from a waiting
+  // blink::ServiceWorkerStatusCode::kOk, there must have been either:
+  // - an active version, which optionally might have activated from a waiting
   //   version (as DidFindRegistrationForFindImpl will activate any waiting
   //   version).
   // - or an installing version.
-  DCHECK(version_ptr);
+  // However, if the installation is rejected, the installing version can go
+  // away by the time we reach here from DidFindRegistrationForFindImpl.
+  if (!version_ptr) {
+    callback_runner->PostTask(FROM_HERE, std::move(failure_callback));
+    return;
+  }
 
   // Note: There might be a remote possibility that |registration|'s |version|
   // might change between here and DidStartWorker, so bind |version| to