Fix possible map::end() dereference in AppCacheUpdateJob triggered by a compromised renderer.

BUG=551044

Review URL: https://codereview.chromium.org/1418783005

Cr-Commit-Position: refs/heads/master@{#358815}
const AppCacheHost* AppCacheHost::GetSpawningHost() const {
  AppCacheBackendImpl* backend = service_->GetBackend(spawning_process_id_);
  return backend ? backend->GetHost(spawning_host_id_) : NULL;
}

const AppCacheHost* AppCacheHost::GetSpawningHost() const {
  AppCacheBackendImpl* backend = service_->GetBackend(spawning_process_id_);
  return backend ? backend->GetHost(spawning_host_id_) : NULL;
}
