Default to returning success in utime.

Restoring the previous behavior of nacl_io regarding utime,
such that it pretends to succeed when a mount doesn't implement utime.

BUG=https://code.google.com/p/chromium/issues/detail?id=414058
TEST=trybots
TBR=binji@chromium.org

Review URL: https://codereview.chromium.org/571773002

Cr-Commit-Position: refs/heads/master@{#294759}
1 file changed