[fuchsia] Use new mx_deadline_after interface

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 209c825e5f1056a4e2892de27fe6060c99a9e9f1..10e2640e922a7e6815ac38b1382d96088c88fc9b 100644
--- a/runtime/vm/os_fuchsia.cc
+++ b/runtime/vm/os_fuchsia.cc
@@ -162,13 +162,12 @@ uintptr_t OS::MaxRSS() {
 
 
 void OS::Sleep(int64_t millis) {
-  mx_nanosleep(millis * kMicrosecondsPerMillisecond *
-               kNanosecondsPerMicrosecond);
+  SleepMicros(millis * kMicrosecondsPerMillisecond);
 }
 
 
 void OS::SleepMicros(int64_t micros) {
-  mx_nanosleep(micros * kNanosecondsPerMicrosecond);
+  mx_nanosleep(mx_deadline_after(micros * kNanosecondsPerMicrosecond));
 }