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}

native_client_sdk/src/libraries/nacl_io/node.cc

@@ -177,7 +177,7 @@ Error Node::Tcsetattr(int optional_actions, const struct termios* termios_p) {
 }
 
 Error Node::Futimens(const struct timespec times[2]) {
-  return EINVAL;
+  return 0;
 }
 
 int Node::GetLinks() {