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}
void DownloadManagerImpl::DownloadRemoved(
    download::DownloadItemImpl* download) {
  if (!download)
    return;

  downloads_by_guid_.erase(download->GetGuid());
  downloads_.erase(download->GetId());
}

void DownloadManagerImpl::DownloadRemoved(
    download::DownloadItemImpl* download) {
  if (!download)
    return;

  downloads_by_guid_.erase(download->GetGuid());
  downloads_.erase(download->GetId());
}
