Modify the Mac sandbox to allow temporary files to be created in a
parent-specified subdirectory of NS_OS_TEMP_DIR. This is similar to the
Windows approach. The parent provides a UUID in a preference which is
used by the content process to form the subdirectory name.
MozReview-Commit-ID: 6BONpfZz8ZI
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extra : rebase_source : ad18e091918356a1a40c13f1453972b4512ad476