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1 - Changed implementation to observe individual DownloadItems rather than
observing an AllDownloadItemNotifier::Observer, because AIDN generates duplicate
updates to the UI.
2 - Changed this class to work with in-progress download manager. This class now
depends on DownloadManagerService to get all downloads.
3 - Changed OfflineContentAggregator to a singleton for android so that it can
 work without a BrowserContext.
4 - TODO : This class is owned by DownloadCoreServiceImpl. Need to fix this
  for service manager mode.

Change-Id: I5741fb0fc9eef05415ab907b23c1fb575ef2203d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1422392
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637978}