Devirtualize PODArena and PODArena::Chunk's destructors
Devirtualize PODArena and PODArena::Chunk's destructors

Devirtualize PODArena and PODArena::Chunk's destructors as these classes are
never subclassed, nor do they have any virtual function. This avoids creating
unnecessary vtables.

BUG=311561

Review URL: https://codereview.chromium.org/44293004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@160629 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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