[base][fuchsia][time] Use C11 timespec for UTC time

UTC time is being removed from the zircon kernel in favor of
user-space clocks, passed to a process through proc_args.

The Fuchsia libc implementation of timespec_get knows where to find
the process-wide clock handle. This implementation is more portable.

Bug: 1119976
Change-Id: Ia5528f51551fedb80059e0bf3960de27512cecf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368013Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Adam Lesinski <adamlesinski@google.com>
Auto-Submit: Adam Lesinski <adamlesinski@google.com>
Cr-Commit-Position: refs/heads/master@{#801507}

base/time/time_fuchsia.cc

@@ -15,11 +15,10 @@ namespace base {
 
 namespace subtle {
 Time TimeNowIgnoringOverride() {
-  zx_time_t nanos_since_unix_epoch;
-  zx_status_t status = zx_clock_get(ZX_CLOCK_UTC, &nanos_since_unix_epoch);
-  ZX_CHECK(status == ZX_OK, status);
-  // The following expression will overflow in the year 289938 A.D.:
-  return Time::FromZxTime(nanos_since_unix_epoch);
+  timespec ts;
+  int status = timespec_get(&ts, TIME_UTC);
+  CHECK(status != 0);
+  return Time::FromTimeSpec(ts);
 }
 
 Time TimeNowFromSystemTimeIgnoringOverride() {