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The bug was operator++ shouln't assume index zero is the front of
the ring's buffer.

This patch also improves test coverage of LazilyDeallocatedDeque.

Change-Id: I8d8a6dd50177a44ab78ef4262a794d6175898864
Reviewed-on: https://chromium-review.googlesource.com/1253982Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595435}