Windows Native Notifications: Fix crash in startup.

The Ready callback passed to SetReadyCallback is executed
on the task runner thread, and not the UI thread.

This causes a crash in startup when launching Chrome.

Also adding a DCHECK to catch this in tests in the
future.

Bug: 820925, 734095
Change-Id: I59d2b152bbc6538d857f3e6884544217b347885e
Reviewed-on: https://chromium-review.googlesource.com/959017
Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542532}

chrome/browser/notifications/notification_display_service_impl.cc

@@ -289,6 +289,7 @@ void NotificationDisplayServiceImpl::ProfileLoadedCallback(
 
 void NotificationDisplayServiceImpl::OnNotificationPlatformBridgeReady(
     bool success) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   if (base::FeatureList::IsEnabled(features::kNativeNotifications)) {
     UMA_HISTOGRAM_BOOLEAN("Notifications.UsingNativeNotificationCenter",
                           success);

chrome/browser/notifications/notification_platform_bridge_win.cc

@@ -480,9 +480,12 @@ class NotificationPlatformBridgeWinImpl
   void SetReadyCallback(
       NotificationPlatformBridge::NotificationBridgeReadyCallback callback) {
     DCHECK(task_runner_->RunsTasksInCurrentSequence());
-    std::move(callback).Run(
-        com_functions_initialized_ && IsToastActivatorRegistered() &&
-        InstallUtil::IsStartMenuShortcutWithActivatorGuidInstalled());
+    bool enabled = com_functions_initialized_ && IsToastActivatorRegistered() &&
+                   InstallUtil::IsStartMenuShortcutWithActivatorGuidInstalled();
+    bool success = content::BrowserThread::PostTask(
+        content::BrowserThread::UI, FROM_HERE,
+        base::BindOnce(std::move(callback), enabled));
+    DCHECK(success);
   }
 
   void HandleEvent(winui::Notifications::IToastNotification* notification,