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When closing tabs, an observer immediately deletes snapshots from the
snapshot cache. This prevents snapshots from reappearing in an undo
close all tabs operation.

This CL enables snapshot restoration on undo close all tabs by extending
the life of a snapshot when closed with a "close all" operation. Note
this is only extended for regular tabs, not incognito tabs.

The extended life is accomplished by marking tabs for deletion during
a close all operation, rather than immediately deleting the image
from the cache. Later, the marked images are either purged from the
cache, or the marked list is cleared (therefore leaving the images in
the cache).

Bug: 804567
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4710bf46d3ebd4ad347563e5f6a6208a68a58209
Reviewed-on: https://chromium-review.googlesource.com/1041545
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556162}