[channel] Use zx_clock_get_monotonic

ZX-2187

Test: no functional change

Change-Id: Ib08b93d98ad84d8864b2566d98c5d62f4dd459a6
diff --git a/system/uapp/channel-perf/main.cpp b/system/uapp/channel-perf/main.cpp
index 12e9d61..2a4bd95 100644
--- a/system/uapp/channel-perf/main.cpp
+++ b/system/uapp/channel-perf/main.cpp
@@ -71,9 +71,7 @@
 
     static constexpr uint32_t big_it_size = 10000;
     uint64_t big_its = 0;
-    zx_time_t start_ns;
-    status = zx_clock_get_new(ZX_CLOCK_MONOTONIC, &start_ns);
-    assert(status == ZX_OK);
+    zx_time_t start_ns = zx_clock_get_monotonic();
     zx_time_t end_ns;
     for (;;) {
         big_its++;
@@ -91,8 +89,7 @@
             assert(r_handles == test_args.handles);
         }
 
-        status = zx_clock_get_new(ZX_CLOCK_MONOTONIC, &end_ns);
-        assert(status == ZX_OK);
+        end_ns = zx_clock_get_monotonic();
         if ((end_ns - start_ns) >= duration_ns)
             break;
     }