Remove use of the deprecated mx_current_time Fuchsia syscall

runtime/vm/os_fuchsia.cc

Index: runtime/vm/os_fuchsia.cc
diff --git a/runtime/vm/os_fuchsia.cc b/runtime/vm/os_fuchsia.cc
index f904926548d1aa03e83e525a1d91c28c916a008f..41a5d96be7d131306a25d70cab7223a74c3c25a2 100644
--- a/runtime/vm/os_fuchsia.cc
+++ b/runtime/vm/os_fuchsia.cc
@@ -75,12 +75,12 @@ int64_t OS::GetCurrentTimeMillis() {
 
 
 int64_t OS::GetCurrentTimeMicros() {
-  return mx_current_time() / 1000;
+  return mx_time_get(MX_CLOCK_MONOTONIC) / 1000;
 }
 
 
 int64_t OS::GetCurrentMonotonicTicks() {
-  return mx_current_time();
+  return mx_time_get(MX_CLOCK_MONOTONIC);
 }