WebApp: Make sure that ManifestUpdateManager works in the provider ready scope.

Make sure that ManifestUpdateManager doesn't cause FinalizeUpdate while
web apps system is not ready.

Note: we should do
tasks_.clear();
in Shutdown to destroy all ManifestUpdateTasks.
That's how we unsubscribe tasks from WebAppUiManagerImpl:

void ManifestUpdateTask::UpdateAfterWindowsClose() {
...
  ui_manager_.NotifyOnAllAppWindowsClosed(
      app_id_,
      base::BindOnce(&ManifestUpdateTask::OnAllAppWindowsClosed,
      AsWeakPtr()));
}

In next CL: Plumb WebAppUiManagerImpl Startup/Shutdown.

Bug: 1084939
Change-Id: I25112df44af57606e38bcc6c12f12f67bd94c8b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212197
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771494}

chrome/browser/web_applications/manifest_update_manager.cc

@@ -36,17 +36,25 @@ void ManifestUpdateManager::SetSubsystems(
 
 void ManifestUpdateManager::Start() {
   registrar_observer_.Add(registrar_);
+
+  DCHECK(!started_);
+  started_ = true;
 }
 
 void ManifestUpdateManager::Shutdown() {
   registrar_observer_.RemoveAll();
+
+  tasks_.clear();
+  started_ = false;
 }
 
 void ManifestUpdateManager::MaybeUpdate(const GURL& url,
                                         const AppId& app_id,
                                         content::WebContents* web_contents) {
-  if (!base::FeatureList::IsEnabled(features::kDesktopPWAsLocalUpdating))
+  if (!started_ ||
+      !base::FeatureList::IsEnabled(features::kDesktopPWAsLocalUpdating)) {
     return;
+  }
 
   if (app_id.empty() || !registrar_->IsLocallyInstalled(app_id)) {
     NotifyResult(url, ManifestUpdateResult::kNoAppInScope);
@@ -82,6 +90,8 @@ void ManifestUpdateManager::MaybeUpdate(const GURL& url,
 
 // AppRegistrarObserver:
 void ManifestUpdateManager::OnWebAppUninstalled(const AppId& app_id) {
+  DCHECK(started_);
+
   auto it = tasks_.find(app_id);
   if (it != tasks_.end()) {
     NotifyResult(it->second->url(), ManifestUpdateResult::kAppUninstalled);

chrome/browser/web_applications/manifest_update_manager.h

@@ -96,6 +96,7 @@ class ManifestUpdateManager final : public AppRegistrarObserver {
   base::Optional<base::Time> time_override_for_testing_;
   ResultCallback result_callback_for_testing_;
 
+  bool started_ = false;
   bool hang_update_checks_for_testing_ = false;
 };