[vm] Avoid deadlocks in SampleBlockProcessor

Processor thread might end up entering a safepoint while holding the
processor's monitor which will make thread calling Cleanup deadlock
while trying to acquire the monitor.

Use SafepointMonitorLocker instead to avoid this. Startup is unlikely
to exhibit this situation but we change it as well for symmetry.

TEST=manually, as reliably hitting this is hard

Change-Id: Ifb88a0418be064450374503bfc485e576a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467801
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>

Author: mraleph

runtime/vm/lockers.h

@@ -282,6 +282,7 @@ class SafepointLocker : public ValueObject {
   typename M::WaitResult Wait(int64_t millis = Monitor::kNoTimeout);
 
   void NotifyAll() { monitor_->NotifyAll(); }
+  void Notify() { monitor_->Notify(); }
 
  private:
   friend class SafepointMonitorUnlockScope;

runtime/vm/profiler.cc

@@ -1896,7 +1896,7 @@ void SampleBlockProcessor::Init() {
 void SampleBlockProcessor::Startup() {
   ASSERT(initialized_);
   ASSERT(processor_thread_id_ == OSThread::kInvalidThreadJoinId);
-  MonitorLocker startup_ml(monitor_);
+  SafepointMonitorLocker startup_ml(monitor_);
   OSThread::Start("Dart Profiler SampleBlockProcessor", ThreadMain, 0);
   while (!thread_running_) {
     startup_ml.Wait();
@@ -1906,7 +1906,7 @@ void SampleBlockProcessor::Startup() {
 
 void SampleBlockProcessor::Cleanup(bool drain /* = false */) {
   {
-    MonitorLocker shutdown_ml(monitor_);
+    SafepointMonitorLocker shutdown_ml(monitor_);
     if (shutdown_) {
       // Already shutdown.
       return;