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Currently DownloadManagerImpl intializes InProgressDownloadManager.
However, when only ServiceManager is running, DownloadManagerImpl will
not be created.
As a result, some other class will initialize InProgressDownloadManager.
And when chrome fully starts, DownloadManagerImpl needs to know whether
InProgressManager has been initialized.
This CL allows InProgressDownloadManager to take more callbacks to
notify when initialized.
So multiple classes can observe the initialization event.

BUG=842245

Change-Id: I5881b10db0f93f98262ab1baebc67621c2743fbe
Reviewed-on: https://chromium-review.googlesource.com/1111027
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569849}