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When traversing the |deferred_page_unloads_| list and handling the
unloads it's possible for new pages to get added to the list which will
invalidate the iterator.

This CL swaps the list with an empty list and does the iteration on the
list copy. New items that are unloaded while handling the defers will be
unloaded at a later point.

Bug: 780450
Change-Id: Ic7ced1c82227109784fb536ce19a4dd51b9119ac
Reviewed-on: https://chromium-review.googlesource.com/758916
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515056}