MachMessageWithDeadline(): 10.6 SDK fix.
MachMessageWithDeadline(): 10.6 SDK fix.

In the pre-C++11 10.6 SDK, std::numeric_limits<>::max() is not marked
constexpr and cannot be used to initialize enum elements.

R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/771183003

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