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Some compilers, including MSVC, don't consider std::atomic as a POD-
compatible type which makes it impossible to include them in persistent
memory when using the object interface -- something which is obviously
desireable.

New "object" management embeds the type directly in the class/struct
definition and runs the default constructor of the type after allocation,
thus making the allocator compatible with a much greater range of types.

BUG=546019

Review-Url: https://codereview.chromium.org/2578323002
Cr-Commit-Position: refs/heads/master@{#442884}