GitLab

Added DownloadStore which is a layer around a leveldb ProtoDatabase that
stores download params, scheduling params, request headers and various
metadata as protos. Callers of DownloadStore expect a list of Entry's,
hence conversion utility methods were added to convert between Entry
and protodb::Entry.

Other details:
1- Moved NoopStore to download/internal/test/
2- Replaced usage of OnceCallback with repeated Callback, since the
proto database expects repeated Callback. In order to change ProtoDatabase
to use OnceCallback, it probably requires a larger change invloving
changing the client sites of ProtoDatabase.

BUG=722705

Review-Url: https://codereview.chromium.org/2881173003
Cr-Commit-Position: refs/heads/master@{#475750}