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Looks like most of the work on this bug was already done. ThreatDetails
is only on the UI thread, and a callback is used after
FinishCollection. These mitigated the immediate problems with
ThreatDetails lifetime. This CL just does the final switch to owning
ThreatDetails with a unique_ptr.

Bug: 777915
Change-Id: Id89b34bb1ee2dbb7a5305822751780ee4ac2e862
Reviewed-on: https://chromium-review.googlesource.com/1183782Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Nathan Parker <nparker@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585217}