desktop-pwas: no SW context use during shutdown

InstallableManager::CheckServiceWorker() now quits
immediately if called after the service worker
context has been destroyed.

Bug: 993666
Change-Id: Idcb336f5532d52e9181c85f71a255579ae0f0cc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1806159
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696703}

chrome/browser/installable/installable_manager.cc

@@ -598,6 +598,9 @@ void InstallableManager::CheckServiceWorker() {
   DCHECK(!worker_->fetched);
   DCHECK(!manifest().IsEmpty());
 
+  if (!service_worker_context_)
+    return;
+
   // Check to see if there is a service worker for the manifest's scope.
   service_worker_context_->CheckHasServiceWorker(
       manifest().scope,