[NaCl SDK] nacl_io: Fix check of nanoseconds in Utimes test

[NaCl SDK] nacl_io: Fix check of nanoseconds in Utimes test

The previous code was checking to make sure the seconds and nanoseconds values
are both greater than or equal to the same values from a time in the past. This
is not true when the nanoseconds value wraps around, so a more complex check is
required.

See a failure here:
http://build.chromium.org/p/client.nacl.sdk/builders/linux-sdk-multi/builds/9092/steps/standalone%20test%3A%20nacl_io_test/logs/stdio

    [ RUN      ] KernelProxyTest.Utimes
    kernel_proxy_test.cc:618: Failure
    Expected: (buf.st_atim.tv_nsec) >= (tm.tv_usec * 1000), actual: 6000 vs 999998000
    kernel_proxy_test.cc:620: Failure
    Expected: (buf.st_mtim.tv_nsec) >= (tm.tv_usec * 1000), actual: 6000 vs 999998000

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/748913004

Cr-Commit-Position: refs/heads/master@{#306915}