Fix some potential data races based on output from tsan.

runtime/vm/thread_interrupter.cc

Index: runtime/vm/thread_interrupter.cc
diff --git a/runtime/vm/thread_interrupter.cc b/runtime/vm/thread_interrupter.cc
index d87411aa1129942c7072156c8f7f8056aacd40e8..1e0d8cf044281cfe41289b6adc953123610470dd 100644
--- a/runtime/vm/thread_interrupter.cc
+++ b/runtime/vm/thread_interrupter.cc
@@ -163,8 +163,8 @@ void ThreadInterrupter::ThreadMain(uword parameters) {
   }
   {
     intptr_t interrupted_thread_count = 0;
-    current_wait_time_ = interrupt_period_;
     MonitorLocker wait_ml(monitor_);
+    current_wait_time_ = interrupt_period_;
     while (!shutdown_) {
       intptr_t r = wait_ml.WaitMicros(current_wait_time_);