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}

components/download/database/download_db.h

@@ -5,6 +5,7 @@
 #ifndef COMPONENTS_DOWNLOAD_DATABASE_DOWNLOAD_DB_H_
 #define COMPONENTS_DOWNLOAD_DATABASE_DOWNLOAD_DB_H_
 
+#include <memory>
 #include <string>
 
 #include "base/callback_forward.h"