Add LevelDB implementation of DownloadDB

This CL:
1. adds the leveldb implementation of DownloadDB.
   New tests are also added to test the db functionality.
2. refactors the components/download/downloader dir.
   The in-progress files are moved to the sub in_progress dir.
   renames components/download/downloader to components/download/database

The BUILD.gn file is moved from original dir, adding holte@ as TBR
BUG=842245
[email protected], [email protected]

Change-Id: I9db4ab257abe1f455b0a9d729e85b521ca664381
Reviewed-on: https://chromium-review.googlesource.com/1058527
Commit-Queue: Min Qin <[email protected]>
Reviewed-by: Bo <[email protected]>
Reviewed-by: David Trainor <[email protected]>
Reviewed-by: Xing Liu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#562117}
42 files changed