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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}