DownloadDB: include <memory> for std::unique_ptr

IWYU: it fixes the build with libstdc++ after dc24395b "Retry initializing download
db on failure"

Bug: 819294
Change-Id: Id8a9bdf56f74d979d4e48747a3a6fbd9c51f59c6
Reviewed-on: https://chromium-review.googlesource.com/1150530
Commit-Queue: José Dapena Paz <jose.dapena@lge.com>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578586}
1 file changed