mac: Add a hack to condition_variable_posix.cc to avoid a crash.
mac: Add a hack to condition_variable_posix.cc to avoid a crash.

The Darwin pthreads subsystem has a bug that can cause the program to crash if a
conditional variable is destroyed after it has been signalled, but before it has
been waited on. This hack is a solution proposed by Apple in its own developer
forums.

BUG=517681

Review URL: https://codereview.chromium.org/1323293005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347736}

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