Download: Adds headers in download_db.h.

This CL adds some includes in download_db.h.

TBR=drtrainor@chromium.org,qinmin@chromium.org

Bug: 842245
Change-Id: I34fcfcf24e933d0be14f76705a65108c975e47e0
Reviewed-on: https://chromium-review.googlesource.com/1153402Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578729}

components/download/database/download_db.h

@@ -7,6 +7,7 @@
 
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "base/callback_forward.h"
 #include "base/optional.h"