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Tasks are deleted on completion (even before crrev.com/c/2507234), this
caused a UAF when a member-object of the AwaitBorealisStartup task was
transitively deleted but its callee.

The chosen solution is to post completion as a separate task rather than
call it from the current completing BorealisTask, we do this is a
mutually-exclusive way so that the deletion of the task is
exclusively-after its completion.

Bug: b/172605172
Change-Id: Iaf23e9ead37196988b24753cef451da318c48436
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521876Reviewed-by: Daniel Ng <danielng@google.com>
Commit-Queue: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#824745}