Early return if a download Id is already used when creating a download

This is protect against download Id overflow and use-after-free
issue.

BUG=958533

Change-Id: I2c183493cb09106686df9822b3987bfb95bcf720
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1591485
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656910}
DownloadManagerImpl::GetServiceConnector() {
  DCHECK_CURRENTLY_ON(BrowserThread::UI);

  service_manager::Connector* connector = GetServiceManagerConnector();
  if (connector)
    return connector->Clone();  // Clone for use on a different thread.
  return nullptr;
}

DownloadManagerImpl::GetServiceConnector() {
  DCHECK_CURRENTLY_ON(BrowserThread::UI);

  service_manager::Connector* connector = GetServiceManagerConnector();
  if (connector)
    return connector->Clone();  // Clone for use on a different thread.
  return nullptr;
}
