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}
bool DownloadManagerImpl::DownloadUrl(
    std::unique_ptr<download::DownloadUrlParameters> params) {
  DownloadUrl(std::move(params), nullptr /* blob_data_handle */,
              nullptr /* blob_url_loader_factory */);
  return true;
}

bool DownloadManagerImpl::DownloadUrl(
    std::unique_ptr<download::DownloadUrlParameters> params) {
  DownloadUrl(std::move(params), nullptr /* blob_data_handle */,
              nullptr /* blob_url_loader_factory */);
  return true;
}
