[vm] Fix race between MarkWorkerAsBlocked/Unblocked.

TEST=vm/dart/isolates/many_isolates_blocked_at_process_run_sync_test
Change-Id: I3efde84cae4a9ea25c75f2f735d5f7b74c22237d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/417561
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/thread.cc

@@ -1373,6 +1373,10 @@ void Thread::UnwindScopes(uword stack_marker) {
 }
 
 void Thread::HandleStolen() {
+  {
+    // To make sure we're sequenced after MarkWorkerAsBlocked.
+    MonitorLocker ml(isolate_group()->thread_registry()->threads_lock());
+  }
   isolate_group()->IncreaseMutatorCount(this, /*is_nested_reenter=*/false,
                                         /*was_stolen=*/true);
 }