Delayed chunks in the unmapper are chunks that should be unmapped but could
potentially still be accessed by the sweeper (page header), hence their
unmapping is delayed.

During TearDown, however, we need to properly unmap those pages, i.e., check
that they can now be unmapped (should always hold) and properly unmap them.

BUG=chromium:656537
R=ulan@chromium.org

Review-Url: https://codereview.chromium.org/2472573003
Cr-Commit-Position: refs/heads/master@{#40698}