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: refs/heads/master@{#347736}
1 file changed