Oilpan: Unpoison orphaned large objects before zapping
Oilpan: Unpoison orphaned large objects before zapping

To zap orphaned large objects, we need to unpoison the area.
Otherwise ASan detects the error, although it seems there has been
no orphaned large objects so far (by accident).

We're already doing that for orphaned normal pages.

This CL also adds a couple of ASSERTs that would be helpful.

BUG=420515

Review URL: https://codereview.chromium.org/1154733002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195782 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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