List of cached directory roots for directory-local variable classes.
Each element in this list has the form (DIR CLASS MTIME).
DIR is the name of the directory.
CLASS is the name of a variable class (a symbol).
MTIME is the recorded modification time of the directory-local
variables file associated with this entry. This time is a list
of integers (the same format as `file-attributes'), and is
used to test whether the cache entry is still valid.
Alternatively, MTIME can be nil, which means the entry is always considered valid.