Best practice for accessing disk files

[rsheeter]

In https://chromium-review.googlesource.com/c/chromium/src/+/5677302/3/content/browser/font_unique_name_lookup/font_unique_name_lookup.cc#229 @danakj notes that use of fontations with mmap'd files on writeable file systems is unsafe.

We should have a documented best practice, and potentially even code, for how to approach this.

• Should people load whole files into memory? - that sucks if you just want a fragment of metadata like the name values in the chrome cl referenced above.
• Should we provide a DiskRef that performs buffered accesss (using a client provided buffer?) or some such?

FontRef is explicitly in-memory, perhaps as a small step forward we should additionally clarify that it is not safe to use with mmap.