background sync: Prepare for service worker UI thread core (pt 2).

This is a follow-up to r689906. It converts code added since then.

Also, this fixes a flaky DCHECK hit when trying to use the
KeepAliveRegistry after shutdown, when running the
VerifyShutdownBehavior test locally. I think this happens when the core
thread is the UI thread because we immediately try to make keep alive
object instead of posting a task to the UI thread to do so.

[ RUN      ] BackgroundSyncBrowserTest.VerifyShutdownBehavior
[FATAL:keep_alive_registry.cc(107)] Check failed: !is_shutting_down_.
...
3 KeepAliveRegistry::Register()
4 BackgroundSyncControllerImpl::BackgroundSyncEventKeepAliveImpl::BackgroundSyncEventKeepAliveImpl()
5 BackgroundSyncControllerImpl::CreateBackgroundSyncEventKeepAlive()
6 content::(anonymous namespace)::CreateBackgroundSyncEventKeepAliveOnUIThread()
7 content::BackgroundSyncManager::DidResolveRegistrationImpl()

Bug: 824858
Change-Id: I5d2ad7878cbd35203fe81fda013f1a839ee89c7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772415Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691621}

chrome/browser/background_sync/background_sync_controller_impl.cc

@@ -12,6 +12,7 @@
 #include "chrome/browser/metrics/ukm_background_recorder_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "components/history/core/browser/history_service.h"
+#include "components/keep_alive_registry/keep_alive_registry.h"
 #include "components/variations/variations_associated_data.h"
 #include "content/public/browser/background_sync_context.h"
 #include "content/public/browser/background_sync_controller.h"
@@ -331,7 +332,8 @@ base::TimeDelta BackgroundSyncControllerImpl::GetNextEventDelay(
 std::unique_ptr<content::BackgroundSyncController::BackgroundSyncEventKeepAlive>
 BackgroundSyncControllerImpl::CreateBackgroundSyncEventKeepAlive() {
 #if !defined(OS_ANDROID)
-  return std::make_unique<BackgroundSyncEventKeepAliveImpl>();
+  if (!KeepAliveRegistry::GetInstance()->IsShuttingDown())
+    return std::make_unique<BackgroundSyncEventKeepAliveImpl>();
 #endif
   return nullptr;
 }

content/browser/background_sync/background_sync_manager.cc

@@ -1548,7 +1548,7 @@ bool& BackgroundSyncManager::delayed_processing_scheduled(
 
 void BackgroundSyncManager::ScheduleOrCancelDelayedProcessing(
     BackgroundSyncType sync_type) {
-  DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  DCHECK_CURRENTLY_ON(ServiceWorkerContext::GetCoreThreadId());
 
   bool can_fire_with_connectivity =
       CanFireAnyRegistrationUponConnectivity(sync_type);
@@ -1568,7 +1568,7 @@ void BackgroundSyncManager::ScheduleOrCancelDelayedProcessing(
 bool BackgroundSyncManager::IsRegistrationReadyToFire(
     const BackgroundSyncRegistration& registration,
     int64_t service_worker_id) {
-  DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  DCHECK_CURRENTLY_ON(ServiceWorkerContext::GetCoreThreadId());
 
   if (clock_->Now() < registration.delay_until())
     return false;