Revert "[fuchsia] Replace zx_clock_get() with zx_clock_get_new()."

This reverts commit 748de1e5.

Reason for revert: Fails compilation (unused variable): https://ci.chromium.org/p/v8/builders/ci/V8%20Fuchsia/10879

Original change's description:
> [fuchsia] Replace zx_clock_get() with zx_clock_get_new().
> 
> zx_clock_get() signature is being replaced to report errors.
> As part of a soft transition, zx_clock_get_new() is introduced with
> the final function signature.
> 
> Bug: chromium:963956
> Change-Id: Ia6d0f4fc560bd9149683011099b064540b54edd6
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1616545
> Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
> Reviewed-by: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61591}

TBR=wez@chromium.org,fdegans@chromium.org

Change-Id: I14a97f6510f6fbd6b3698a3549def8feb128f0ff
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:963956
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1617241Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61593}

src/base/platform/platform-fuchsia.cc

@@ -150,10 +150,7 @@ void OS::SignalCodeMovingGC() {
 int OS::GetUserTime(uint32_t* secs, uint32_t* usecs) {
   const auto kNanosPerMicrosecond = 1000ULL;
   const auto kMicrosPerSecond = 1000000ULL;
-  zx_time_t nanos_since_thread_started;
-  zx_status_t status =
-      zx_clock_get_new(ZX_CLOCK_THREAD, &nanos_since_thread_started);
-  DCHECK_EQ(status, ZX_OK);
+  const zx_time_t nanos_since_thread_started = zx_clock_get(ZX_CLOCK_THREAD);
 
   // First convert to microseconds, rounding up.
   const uint64_t micros_since_thread_started =