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The logic in loading the download history is quite complicated today.
When DownloadManagerImpl is created, it first loads the download from
in-progress manager. And then it starts to load the downloads from
History DB. That's causing several issues:
1. Due to the mismatch between history DB and in-progress DB, DownloadManagerImpl
need to resolve the conflicts when history DB is loaded. Normally it just
takes the information from in-progress DB. However, due to the latency during
database commit, history DB might contain fresher information than the
in-progress DB if the download is in a terminating state (Cancelled/Completed).
And that's causing crbug/898859 as an extra OnDownloadCreated() call is called
for those terminating downloads.

2. DownloadHistory used to call CreateDownloadItem() and expect a reentrant
OnDownloadCreated() call when loading history downloads. However, this may
no longer happen as OnDownloadCreated() may have already been called when
loading the in-progress DB. So DownloadHistory have to handle the case that
there is no reentrant call, which makes the code hard to understand.

3. DownloadHistory might get a OnDownloadCreated() call even before history
is loaded. And some of the downloads might have already existed in the
history DB. This will cause a failed sql insert LOG in the console. Although
the behavior is expected, it is not quite clean.

This CL fixes all above issue by loading the in-progress DB records and the
history DB records at the same time. Once CreateDownloadItem() is called,
DownloadManagerImpl will find the corresponding in-progress download and return
it to DownloadHistory, and calling OnDownloadCreated() at the same time.

BUG=898859

Change-Id: I0f1da0ecea99e72e8c5507ea005c56261a46f3cd
Reviewed-on: https://chromium-review.googlesource.com/c/1318692Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605900}