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Holding space items will be persisted to profile prefs. Note that we
don't persist Downloads as those have a separate persistence storage.
Also note that we don't yet persist images. Ideally, we shouldn't need
to.

Known deficiences:
- Doesn't yet restore downloads
- Doesn't yet restore images

Bug: 1119496
Change-Id: I74f929c8bb802a15e6771dc58ea68ec20380fe26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367701Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#801136}