Our vendored gperftools currently can't enable the debug mode when building on OS X due to https://code.google.com/p/gperftools/issues/detail?id=581

While we have requested that gperftools release stable update that includes the fix (https://code.google.com/p/gperftools/issues/detail?id=616), they have yet to do so.

We should cherry pick the fix they made for 581 into our gperftools so that we can re-enable debug mode on OS X.

Later if they upgrade gperftools we can replace.