Make DownloadHistory observe manager, items

      Rip out half of DownloadManagerDelegate.
Make DownloadManager create persisted DownloadItems one at a time and return them to DownloadHistory.
Move DownloadPersistentStoreInfo from content to chrome.
Kill DownloadDatabase::CheckThread(). (Leftover from 85408.)
Change DownloadDatabase::RemoveDownloads() to take an explicit set of db_handles.
Merge DownloadDatabase::UpdateDownload[Path]().

After this CL, I'll send out another one to remove the usage of CancelableRequest from the downloads-specific HistoryService APIs.

BUG=154309

Review URL: https://chromiumcodereview.appspot.com/10915180

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@168670 0039d316-1c4b-4281-b951-d872f2087c98