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Peek() always takes a reference to an existing slot in a table. Therefore,
Peek() does not take a nullptr_t value, since nullptr_t is not stored in
the backing buffer.

BUG=715430
R=tzik@chromium.org

Review-Url: https://codereview.chromium.org/2844583004
Cr-Commit-Position: refs/heads/master@{#467920}