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Note that this CL also updates the lifecycle of the holding space keyed
service. Previously it was created with browser context but is now
delayed until start of the user session.

Also note modifications to DownloadManagerUtils to address tests broken
by this CL which assumed DownloadManagerImpl would not be created until
after tests bodies executed.

Bug: 1155387
Change-Id: I7e0f226cb275465c72eb8d63f1348b44f5412685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572694
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838744}