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We already started removing cancelled and non-resumable download from history
DB. And we should do the same for download DB.
This CL moves all the logic of clearing those download into DownloadManagerImpl.
Here is the work flow:
1. when in-progress download manager is loaded, clear all the non-resumable
downloads.
2. When DownloadHistory loads all the history items, a nullptr will be
returned if the history item should be cleaned, and history DB will remove
the item.
3. After DownloadHistory finished loading, in-progress download manager will
remove all the non-resumable downloads, so that we are guaranteed that changes
in Download DB are propagated to DownloadHistory

This CL also fixes issues that cancelled download in Download DB are not
propagated to history DB.

BUG=851650

Change-Id: Ieeec319cd429ecca7a327e463e2a869dd00a7f4e
Reviewed-on: https://chromium-review.googlesource.com/c/1285318Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601367}