Fix race discovered by Valgrind

runtime/vm/profiler_win.cc

Index: runtime/vm/profiler_win.cc
diff --git a/runtime/vm/profiler_win.cc b/runtime/vm/profiler_win.cc
index 02a7af2afe9976387d8d8e5ca350ecc5e5a839f7..3bc9e41680054f9a739986fc77fa94dcabd5e6df 100644
--- a/runtime/vm/profiler_win.cc
+++ b/runtime/vm/profiler_win.cc
@@ -142,10 +142,10 @@ void ProfilerManager::ThreadMain(uword parameters) {
   if (FLAG_trace_profiled_isolates) {
     OS::Print("ProfilerManager Windows ready.\n");
   }
-  thread_running_ = true;
   {
     // Signal to main thread we are ready.
     ScopedMonitor startup_lock(start_stop_monitor_);
+    thread_running_ = true;
     startup_lock.Notify();
   }
   ScopedMonitor lock(monitor_);
@@ -157,10 +157,10 @@ void ProfilerManager::ThreadMain(uword parameters) {
   if (FLAG_trace_profiled_isolates) {
     OS::Print("ProfilerManager Windows exiting.\n");
   }
-  thread_running_ = false;
   {
     // Signal to main thread we are exiting.
     ScopedMonitor shutdown_lock(start_stop_monitor_);
+    thread_running_ = false;
     shutdown_lock.Notify();
   }
 }