Add strnlen() wrapper and its test

Add strnlen() wrapper and its test.

TEST=util_test strnlen.strnlen
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/af0cfd5a57cd

[Mark Mentovai, 2014-08-28 18:13:15]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Robert Sesek, 2014-08-28 18:33:01]

I'm surprised you didn't just conditionally declare this in the global namespace
for SDKs < 10.7. LGTM, though,

[Mark Mentovai, 2014-08-28 18:37:45]

Yes, that’s an alternative. strnlen() is new and support is spotty across a
variety of systems. I’d be duplicating that in a bunch of platforms’ compat
directories.

The same argument applies to some other things in util/stdlib. Since they’re all
consolidated there, if we decide to change the approach and move them into
compat, it should be easy.

[Mark Mentovai, 2014-08-28 18:38:31]

Committed patchset #1 (id:1) manually as af0cfd5a57cd (presubmit successful).